Mortimer, in this play, is an old man. He is the same Mortimer that appears in Shakespeare's Henry IV part 1. The touching scene between Mortimer and York is primarily one of exposition, explaining to the audience the historical claim York has on his titles, and setting up his later attempt to take the crown in 2 and 3 Henry VI.
The following information is quoted from the Oxford Dictionary of National Biography, written by R. A. Griffiths
Mortimer, Edmund (V), fifth earl of March and seventh earl of Ulster (1391–1425), magnate and potential claimant to the English throne, was the last earl of March of the family of Mortimer of Wigmore; his descent from Edward III blighted his career and threatened the usurping Lancastrian dynasty after 1399. According to a contemporary family history (Chicago University Library, MS 224), he was born on 6 November 1391 in the New Forest, the elder son of Roger (VII) Mortimer, fourth earl of March (1374–1398), and Eleanor (d. 1405), daughter and coheir of Thomas Holland, earl of Kent. After his father's death Edmund and the Mortimer estates in England, Wales, and Ireland were at Richard II's disposal; Edmund's mother's dower third was valued at £1242 a year.
Following Richard II's deposition by Henry Bolingbroke in October 1399 rebels periodically publicized Mortimer's claim. Though plausible, this was weakened by descent through a female—his father's mother, Philippa, daughter of Lionel of Clarence, Edward III's second surviving son—and by Edmund's youth; nevertheless, Henry IV placed this potential rival under strict supervision. Mortimer's estates were transferred, at first, to the Percys (1399–1401), while Edmund and his younger brother Roger (d. c.1413) lived as royal wards, mostly at Windsor. When Henry IV campaigned in Wales in 1402, as a precaution the boys were transferred temporarily to Berkhamsted Castle, in the care of the Lancastrian knight Hugh Waterton. The wisdom of this was demonstrated when the boys' uncle, Sir Edmund (IV) Mortimer (d. 1408/9), who had been captured by Owain Glyn Dŵr in June 1402, went over to the Welsh rebels, announcing that his nephew Edmund was Richard II's rightful heir. The name of Mortimer struck a chord in Wales, not simply as that of the greatest of marcher families, but also because the earls of March descended from Llywelyn ab Iorwerth (d. 1240), prince of Gwynedd. In the following year the Percys also rebelled ostensibly in Mortimer's interest; the earl of Northumberland's heir, Henry Percy (Hotspur), was married to Edmund's aunt. In May 1405 the Scrope revolt in Yorkshire publicized the Mortimer claim once again. In February of that year a daring attempt had been made to abduct the two boys from Windsor Castle and convey them to Wales, probably to join Glyn Dŵr and Sir Edmund Mortimer, who were allegedly negotiating a tripartite alliance with the earl of Northumberland whereby Sir Edmund (perhaps on behalf of his nephew) would receive the southern part of the English realm. Constance, Lady Despenser, was responsible for the abduction, encouraged by her brother Edward, duke of York, who was briefly arrested. While making for Cardiff the party was apprehended near Cheltenham, Lady Despenser was arrested, and the boys returned to Windsor. More secure custody was recommended by the council, and in February 1406 they were put in the charge of another of Henry IV's retainers, Sir John Pelham, at Pevensey Castle.
Mortimer's estates experienced difficulties in these years: the earl of Orkney attacked Ulster c.1401, and although Glyn Dŵr's forces may have spared the lordship of Denbigh while it was in Hotspur's custody, c.1401–2, other of Mortimer's Welsh estates were devastated from 1403, thereby contributing to his later financial problems. As he grew older (and Henry IV felt less threatened), Edmund's future merited special consideration. On 24 February 1408 his marriage was granted to Queen Joan, on condition that he could marry only with the king's assent and the council's advice. A year later, he and his brother were placed in the care of their cousin, Prince Henry; the younger Mortimer may have died soon after the prince became king in 1413. Their sisters, Anne and Eleanor, in the care of their mother who in 1400 married Edward Charlton, Lord Powys, fared less well after Countess Eleanor died in 1405, for they were shortly described as destitute and needed £100 per annum for themselves and their servants.
The death of Henry IV may have prompted renewed murmurings about Edmund Mortimer's claim. Nevertheless, Henry V released him from custody and knighted him on the eve of his coronation (9 April); in parliament in June he was declared of age and allowed to inherit his estates. Yet suspicions remained, and on 18 November 1413 Mortimer concluded a recognizance for 10,000 marks with the king to ensure his loyalty. Moreover, his privy purse expenses for 1413/14 show the 23-year-old earl still in the king's company. His gift to the king about 1414, from one bachelor to another, of a Welsh boy (aged nine) and girl (aged seven) and the child they produced—‘an event which caused widespread amazement’ (Chronicle of Adam of Usk, 245)—may have seemed like misplaced irony. Mortimer may already have identified his own intended bride, for in February 1415 he secured a dispensation from Pope John XXIII to enable him to marry ‘a fit woman related to him in the third degree of kindred or affinity’ because he ‘desires to have children, but being related to divers magnates cannot find a wife suitable to his rank whom he can marry without papal dispensation’ (CEPR letters, 6.456). This woman was Anne (d. 1432), daughter of Edmund, earl of Stafford, and herself descended from Edward III. Henry understandably disapproved and imposed on Mortimer the large fine of 10,000 marks, which became a significant element in Henry's war finances.
Mortimer was involved in preparations for the invasion of France, and on 24 July 1415 he witnessed the king's will at Southampton. But already the ‘Southampton plot’ was laid, notably by Edmund's brother-in-law, Richard, earl of Cambridge, with the aim of taking Edmund to Wales to proclaim him king. At Portchester on 31 July Mortimer, who rarely took the initiative in the movements in his name, surprised the conspirators by revealing all to the king. Henry accepted his protestations of loyalty and placed him on the commission (5 August) that tried and condemned his erstwhile accomplices; on 7 August he was granted a pardon.
Bound to the king and deeply in debt, Mortimer participated in Henry V's Normandy campaigns, usually in the king's company. In August 1415 he led 59 men-at-arms and 160 archers across the channel, but during the siege of Harfleur he was invalided home suffering from dysentery. In 1416 he joined Bedford to relieve Harfleur; in 1417, as well as taking a larger retinue (100 lances and 300 archers) to France, he had charge of a fleet to patrol the seas. As the king's lieutenant in Normandy (June 1418) he was responsible for the Cotentin, Domfront, and Caen, and he joined the king for the taking of Rouen; but he received no significant war profits. When Henry and Queen Catherine travelled to England in February 1421, Edmund Mortimer accompanied them and bore the queen's sceptre at her coronation. He returned to France with Henry V in June, and was at the siege of Meaux when the king fell mortally ill.
After Henry V's funeral at Westminster, Mortimer was appointed one of the new king's councillors in November 1422. It may be that the accession of the nine-month-old Henry VI threw into sharper relief the activities of Mortimer's likely kinsman, Sir John Mortimer, a suspected traitor who was executed in 1424 for plotting a rising in Wales to make Edmund king. Certainly, in 1424 Henry VI's uncle, Humphrey, duke of Gloucester, expressed unease at the size of Edmund Mortimer's retinue—larger than Henry V had allowed—and this may explain why Mortimer was appointed the king's lieutenant in Ireland in March 1423, though a visit to his ravaged estates there (they included the lordships of Trim and Connacht) was long overdue. Ships were commissioned for his journey in 1423–4, and Mortimer was paid most of his first year's stipend, but not until the autumn of 1424 did he set sail. On 18 January 1425, at his castle of Trim, he died of plague. His body was conveyed to England for burial in the Augustinian friary at Clare, Suffolk, which his ancestors had patronized. Mortimer had no children of his own and his heir was Richard, duke of York (1411–1460), the son of Mortimer's sister Anne and Richard, earl of Cambridge, whose own descent from Edward III's fourth son fortified York's regal connection. During York's minority, the dukes of Gloucester and Exeter enjoyed most of the Mortimer estates, with dower provision made (after some difficulty) for the earl's widow Anne, who shortly married (by 1427) John Holland, earl of Huntingdon. She died in September 1432. The family chronicle describes Edmund Mortimer as ‘severe in his morals, composed in his acts, circumspect in his talk, and wise and cautious during the days of his adversity’. He earned the epithet the Good because of his kindness, and he was noted for his devotion to God, his discretion, and generosity (Dugdale, Monasticon 6.355, from Chicago University Library, MS 224). This is likely to be a sympathetic gloss on the character of a rudderless noble whose lineage placed him at the mercy of others.