I've previously written posts about the last de Warenne earl of Surrey, John de Warenne (30 June 1286-29 June 1347), his disastrous marriage to Edward II's French niece Jeanne de Bar (c. 1295/96-1361), and his nine known illegitimate children. Here's a post about the earl's last mistress, Isabelle Holland.
The key piece of evidence identifying Isabelle as Earl John's mistress is his will, dated at his Yorkshire castle of Conisbrough on Sunday, 24 June 1347 five days before he died, wherein John refers to her as Isabelle de Holand, ma compaigne. This literally means 'my consort' or 'my wife'. In point of fact, Isabelle was not and could not have been John's wife, as he had married Jeanne de Bar on 25 May 1306 in the presence of her grandfather, Edward I. He was then nineteen going on twenty and Jeanne was about ten or eleven. King Edward had offered John his granddaughter's marriage on 15 May 1305, and John gladly accepted, though their marriage appears to have failed as early as August 1309, and by August 1310 John already had at least one illegitimate child.  In February 1316, having fathered several more illegitimate children in a long-term relationship with his mistress Maud Nerford, John de Warenne began to make strenuous though unsuccessful attempts to annul his marriage to Jeanne de Bar, so that he could marry Maud instead. He and Maud claimed that they had already been pre-contracted to marry when John wed Jeanne in 1306, which of course was nonsense, and failed. By c. 1320, John's relationship with Maud Nerford had ended, and he "removed her from his heart and ousted her from his company".  Given the nine illegitimate children named in his will and in other sources, at least one of whom was born in Conisbrough Castle, he must have embarked on another relationship or several.
Earl John's relationship with Isabelle Holland began sometime before early 1344: on 26 February 1344, Pope Clement VI ordered him to "receive and treat with marital affection" his wife Jeanne de Bar, and in April/June that year there are more papal letters indicating that John was attempting, again, to have his marriage annulled. This time, he claimed that a) the 1306 papal dispensation issued to him and Jeanne for consanguinity was invalid, and that b) he had had an affair with his wife's aunt Mary, nun of Amesbury Priory (1279-1332, Edward I's fourth daughter), before he married Jeanne.  Again, John's attempts failed - the queens-consort of England and France, Philippa of Hainault and Jeanne of Burgundy, lobbied the pope on the countess of Surrey's behalf - and when he died, he was still married to Jeanne de Bar and had been for forty-one years. Given the wording in his will of June 1347, however, John was convinced that Isabelle Holland was his rightful wife, whatever anyone else said.
Isabelle is not specifically identified in her lover's will, but the names of other people who also appear in the will make it apparent who she was, and an entry on the Patent Roll of December 1346 also identifies her. She was one of the daughters of Sir Robert Holland or Holand (born 1270s), a knight of Lancashire who became the steward and close associate of Edward II's cousin Thomas, earl of Lancaster and Leicester. Edward II imprisoned Robert after the Contrariant rebellion of 1321/22, though he escaped from prison in Northampton at an unknown date after 23 July 1326; Queen Isabella pardoned him in 1327; and in October 1328, he was waylaid in a wood in Essex and beheaded by a group of Lancastrians disgruntled at what they saw as Robert's betrayal of Earl Thomas in 1322. They sent his head to Thomas's brother and heir, Henry of Lancaster. Sir Robert Holland's career is fascinating; I'll try to write a post on him at some point. 
Sometime before 13 May 1306, Earl Thomas of Lancaster had arranged Sir Robert Holland's highly advantageous marriage to Maud la Zouche.  Maud was the co-heir, with her elder sister Ellen or Elena, of their father Alan la Zouche, who owned lands in a few counties in the Midlands and south of England. Alan was born in 1267 and died in 1314, and his second daughter Maud was born in about 1288 or 1290; in April/May 1314, she was either twenty-four or twenty-six.  She was a good few years younger than her husband Robert, and somewhat younger than the man who would become her daughter's lover: John de Warenne, earl of Surrey, was born on 30 June 1286. Maud la Zouche Holland outlived her husband Robert by twenty years and died in May 1349. She was called Dame Maude de Hollande in the earl of Surrey's will of June 1347, and the earl left her four mares (iiij jumentz) from his stud-farm in Sussex and appointed her as one of his executors. 
Robert Holland and Maud la Zouche's eldest son and heir was also named Robert, and was said to be sixteen on 1 December 1328 and seventeen or "seventeen and more" in early January 1329, placing his date of birth around 1311/12 (though in July 1349, he was supposedly "aged thirty years and more at Easter last").  Their second son was named Thomas, which might mean that the earl of Lancaster was his godfather, and Thomas Holland (d. late 1360) made a brilliant marriage to Edward II's niece Joan of Kent, later countess of Kent in her own right and princess of Wales; Thomas's children, born in the 1350s, were the older half-siblings of King Richard II. Both Thomas and his younger brother, Sir Otto or Otho Holland, were among the founder members of the Order of the Garter in 1348 (Thomas was number thirteen and Otto was the twenty-second). Robert (d. 1373), the eldest of the Holland brothers and their parents' heir, is more obscure than his younger brothers Thomas and Otto Holland, who both played important roles in Edward III's wars in France and were known as valiant and brilliant knights. Another Holland brother, Alan, appears on the Patent Roll on 15 November 1321 with his older brothers Robert and Thomas.  Otto Holland is not mentioned in that entry, and surely would have been, had he been born by then (as it talks of the "heirs male" of Robert Holland Sr and Maud la Zouche), so it would seem that he was born after November 1321 and was the fourth Holland brother.  John de Warenne, earl of Surrey, appointed Sir Thomas Holland as one of his executors in 1347, and left items - various pieces of equipment for destriers - to Sir Robert Holland Jr and Sir Otto Holland (Monsire Otes de Holande). Thomas Holland and his brother-in-law Sir John Darcy witnessed a quit-claim of Earl John's on 1 April 1346. 
The Genealogics website mentions four daughters of Robert Holland Sr (d. 1328) and Maud la Zouche (d. 1349): Eleanor or Alianore, who married Sir John Darcy and died in or before 1341; Maud, who married firstly John, Lord Mowbray (1310-61) in or before 1319, which was annulled, and secondly Sir Thomas Swynnerton (1313-61); Margaret, who gave birth to her son Roger, Lord la Warre (or Ware or Warr) in November 1326; and Elizabeth, who married Sir Henry FitzRoger (born 1318) before 1340 and had a son John, and died in 1387. The website does not mention Isabelle Holland as one of Robert and Maud's daughters, but an entry on the Patent Roll of 12 December 1346, relating to John de Warenne's attempts to pass on some of his lands to her after his death, calls her "Isabel de Holande, daughter of Robert de Holande, begotten on the body of Maud, late his wife". It could hardly be clearer that she was indeed one of their daughters.  An unnamed "daughter of Robert de Holand" is mentioned on 26 February 1322, when she was sent to the Tower of London as a hostage, with the children of other Contrariants. 
I don't know how Isabelle Holland fits into the birth order of the nine Holland/la Zouche children. I did wonder if she might be the same person as her sister Elizabeth FitzRoger, as the names Isabel(le) and Elizabeth were often interchangeable, but Elizabeth married Henry FitzRoger before 1340, so she can't be. Margaret Holland gave birth to her son Roger la Warre as early as 1326, so she must have been one of the eldest Holland children, older than her brother Robert and surely born in or before 1310 (the Holland parents married in or before 1306). Elizabeth's son John FitzRoger was born after 1345 and perhaps as late as the early 1350s, and Elizabeth herself lived until 1387, so she would certainly seem to be one of the youngest Holland children, as was her brother Sir Otto. As Isabelle was not yet married when she began a relationship with the earl of Surrey in c. 1343, she would also appear to have been one of the younger Holland children. Sir Robert Holland was imprisoned in the spring of 1322 and remained in captivity for well over four years, and one assumes he was not allowed conjugal visits, with the result that Maud la Zouche is unlikely to have borne any children for some years after 1322. She may, however, have become pregnant again in c. 1327/28 in the period between her husband's escape from prison in Northampton sometime after 23 July 1326 and his murder. Born c. 1288/90, Maud was in her late thirties or forty years old when Robert was beheaded in October 1328, and although she was already a grandmother to Roger la Warre she was still of an age to bear another child in 1327/28.
Although it is impossible to establish Isabelle Holland's date of birth, she was several decades younger than John de Warenne, earl of Surrey, who was about two or four years older than her mother. Isabelle was also younger than John's oldest illegitimate children, one of whom was born sometime before August 1310, and another two of whom were born before 1316. John's much earlier mistress Maud Nerford was the daughter of Sir William Nerford of Norfolk and the niece of William, Lord Ros of Helmsley in Yorkshire, and Isabelle also had a noble background. She was a great-granddaughter of Nicholas, Lord Segrave (d. 1295) and a great-niece of Gilbert Segrave, bishop of London (d. 1316), and a great-great-great-granddaughter of Roger de Quincy, earl of Winchester (d. 1264). It's impossible to know how many women Earl John had relationships with, and how many mothers his children had, though the two women for whom he attempted to annul his marriage to Jeanne de Bar both came from noble families, and he deemed both Maud and Isabelle of suitably illustrious birth and rank for him to marry.
John and Isabelle's relationship would appear to have been well established by early 1344, when the earl began to entertain serious thoughts of marrying his lover. In 1347 John appointed Isabelle's mother and her brother Thomas as his executors, and left items to her other brothers Robert and Otto, and was evidently on very good and very close terms with the Holland family. He left Isabelle numerous items including all his beds, all the vestments for his chapel, his "gold ring with the good ruby", another five gold rings in a gold eagle, all his vessels of plain silver, and half of his livestock (la moyte de mon estor). Furthermore, she was to receive all of John's goods and chattels except those which went to pay his debts or which he had bequeathed elsewhere. Earl John died on 29 June 1347, five days after making his will and the day before his sixty-first birthday. At his inquisition post mortem held in Sussex on 10 July 1347, the jurors stated that Countess Jeanne "a year and more ago, crossed the sea with the king's licence...whether she is surviving or not the jurors know not." The Wiltshire and Dorset jurors also admitted that they had no idea whether she was still alive, though in fact Jeanne outlived John by fourteen years. She had surely travelled to her native county of Bar in eastern France; her nephew Count Henri IV (b. 1321) died in 1344 and was succeeded by her young great-nephew Edouard II, and subsequently by Henri's second son Robert, the first duke of Bar. 
It seems unlikely that Isabelle Holland was the mother of any of Earl John's children, except perhaps Katherine, who appears in his will simply as "Katherine my daughter" (Katerine ma fille). His six known sons were far too old to be Isabelle's children, as was his daughter Isabel, already a nun at Sempringham Priory in Lincolnshire in 1347. John's other daughter was named in his will as Johanne de Basyngg, so either she had married a man called Basing or her mother's name was Basing. In the fourteenth century, people born out of wedlock often used their mother's last name, though having said that, at least five of John de Warenne's sons (Sir William, Prior William, John, Thomas and Sir Edward) used the name de Warenne.
Sometime before December 1346, John attempted to settle some of his lands on Isabelle, to pass to her after his death: the Yorkshire castles of Conisbrough and Sandal, and eight manors also in Yorkshire, including Wakefield, Halifax and Dewsbury. John's nephew and heir Richard, earl of Arundel, born c. 1313 as the son of John's sister Alice (1287-1338), however, refused to accept these arrangements. He petitioned Edward III complaining that he would be disinherited. Edward III agreed and revoked John's grant, though in fact Conisbrough Castle and John's other Yorkshire lands ended up passing to the king's fourth son Edmund of Langley, later earl of Cambridge and first duke of York (1341-1402). Edmund was almost certainly Earl John's godson; in his will John left valuable items to King Edward, Queen Philippa, their eldest son Edward of Woodstock, prince of Wales, and Edmund of Langley. None of the many other royal children were mentioned in it. 
Countess Jeanne, John de Warenne's legal widow despite all his efforts over the decades, was granted her large dower in Dorset, Somerset, Wiltshire and Surrey on 24 August 1347.  It probably goes without saying, as John was pretending that he was married to Isabelle, that poor Jeanne de Bar was not mentioned in her husband's will. She was the only daughter of Edward I's eldest surviving daughter Eleanor (1269-98), so was a pretty important person, not that you'd know it from Earl John's treatment of her. They just seem to have been totally incompatible, and given that there was a rift between them as early as August 1309 when Jeanne can't have been more than thirteen or fourteen, I wonder if they ever had an intimate marital relationship at all. They certainly had no children.
Unfortunately, this post peters out lamely at this point, as I have no idea what happened to Isabelle Holland after John de Warenne's death in 1347. She was probably not yet thirty and perhaps a few years younger than that, and if she was conceived and born after Robert Holland's escape from prison after July 1326, she might not even have been twenty years old (!!). She was left many valuable posessions by the earl, and had a noble background and excellent connections: her sister-in-law Joan of Kent became countess of Kent in her own right on her brother's death in late 1352, and in 1361 married Edward III's eldest son Edward of Woodstock. She might therefore have made a good marriage, even if she did not become countess of Surrey, or at least chatelaine of Conisbrough and Sandal, as John de Warenne had wished. Possibly Isabelle does appear on record somewhere after 1347, and if I ever find the time I'll try to research the matter more. I hope she had a long and happy life.
1) Calendar of Chancery Warrants 1244-1326, p. 296; Calendar of Close Rolls 1302-7, p. 321; Calendar of Patent Rolls 1307-13, pp. 330, 594; CPR 1313-17, pp. 528-9.
2) CPR 1313-17, pp. 12, 401, 434, 528-9; The National Archives SC 8/87/4348.
3) Calendar of Papal Letters 1342-62, pp. 116, 169, 173.
4) CPR 1345-48, p. 221, identifies Isabelle; CCR 1323-27, p. 592, is Edward II's order of 23 July 1326 to move Robert Holland from imprisonment at Warwick Castle to Northampton; CPR 1327-30, p. 17, is Robert's pardon for breaking prison at Northampton.
5) Feet of Fines, Berkshire, CP 25/1/9/38, no. 10, dated 13 May 1306, talks of "Robert de Holond and Maud his wife" when the manor of Denford was given to them with remainder to Maud's father Alan la Zouche.
6) Calendar of Inquisitions Post Mortem 1307-17, no. 458. Her younger sister Elizabeth la Zouche, about twenty in 1314, was a nun.
7) John's will is printed in Testamenta Eboracensia, vol. 1, pp. 41-5.
8) CCR 1327-30, pp. 348, 491; CIPM 1327-36, no. 156; CIPM 1347-52, no. 199.
9) CPR 1321-24, p. 40.
10) Sir Otto Holland died in September 1359, and his eldest brother Robert was his heir: CIPM 1352-60, no. 557.
11) East Sussex Record Office, AMS4952/3.
12) CPR 1345-48, p. 221.
13) CPR 1321-24, p. 75.
14) CIPM 1347-52, nos. 54-5, John's IPM; also CPR 1345-48, p. 226.
15) CPR 1345-48, p. 221; CPR 1348-50, pp. 161, 164; Calendar of Charter Rolls 1341-1417, p. 63.
16) CCR 1346-49, pp. 314-17; CIPM 1361-65, no. 215.