Roger Mortimer, born April 11, 1374, at Usk in Monmouthshire, is the eldest son of Edmund Mortimer, 3rd Earl of March, by his wife Philippa Plantagenet, who as the daughter of Lionel of Antwerp, 1st Duke of Clarence, and granddaughter of King Edward III, had a claim to the crown which she passed on to her children.

He has a younger brother, Edmund Mortimer, and two sisters, Elizabeth, who has married Henry 'Hotspur' Percy, and Philippa (1375–1401), who will marry firstly John Hastings, 3rd Earl of Pembroke, killed in a tournament at Woodstock Palace in 1389, secondly Richard de Arundel, 11th Earl of Arundel, who will be beheaded in 1397, and thirdly, Sir Thomas Poynings.

Roger Mortimer's mother, Philippa, died on or before January 5, 1382, and was buried at Wigmore Abbey.

His father, said to have caught cold crossing a river in winter, had died at the Dominican friary at Cork in Munster on December 27, 1381, leaving his son to succeed to a title and extensive estates at only six years of age.

Mortimer's estates in England and Wales are on December 16, 1383, granted, for four thousand pounds per annum, to a consortium consisting of Mortimer himself, the Earls of Arundel, Northumberland, and Warwick, and John, Lord Neville.

The guardianship of Mortimer's person had initially been granted to Arundel, but at the behest of King Richard's mother, Joan of Kent, in August 1384 Mortimer's wardship and marriage had been granted, for six thousand marks, to Joan's son, Thomas Holland, 2nd Earl of Kent, who is Richard's half-brother, and on or about October 7, 1388, Mortimer had married Kent's daughter, Eleanor Holland, who is Richard's half-niece.

Mortimer does homage and was on June 18, 1393, granted livery of his lands in Ireland, and on February 25, 1394, of those in England and Wales .

King Richard has no issue, and Mortimer, a lineal descendant of Edward III, is next in line to the throne and married to his half-niece.

George Edward Cokayne, in The Compete Peerage (1932) states that in October 1385 Mortimer was proclaimed by the King as heir presumptive to the crown.

However, according to R.R. Davies in his entry for Mortimer in the Oxford Dictionary of National Biography (1994) the story that Richard publicly proclaimed Mortimer as heir presumptive in Parliament in October 1385 is baseless, although contemporary records indicate that his claim was openly discussed at the time.

He was knighted on April 23, 1390, by the King.

Mortimer after he came of age spent much of his time in Ireland.

King Richard had first made Mortimer his Lord Lieutenant of Ireland on January 24, 1382 when he was a child of seven, with his uncle, Sir Thomas Mortimer, acting as his deputy.

The King had reappointed Roger Mortimer as his lieutenant in Ireland on July 23, 1392, and in September 1394 Mortimer accompanies the King on an Irish expedition in response to interclan strife affecting English administration.

Richard engages in little fighting, but successful negotiations gain him the submission of fifty Irish chiefs and result in his knighting of five Irish kings before he departs the following year.