She was born at Eltham Palace in Kent on 16 August 1355, the only child of Lionel of Antwerp, 1st Duke of Clarence, and Elizabeth de Burgh, 4th Countess of Ulster. Her father was the third son, but second son to survive infancy, of King Edward III of England and Philippa of Hainault.
Philippa married Edmund Mortimer, 3rd Earl of March, in 1368 at the age of thirteen in the Queen's Chapel at Reading Abbey, an alliance that would have far-reaching consequences in English history. Her cousin, King Richard II, remained childless, making Philippa and her descendants next in line to the throne until his deposition. In the Wars of the Roses, the Yorkist claim to the crown was based on descent from Edward III through Philippa,, her son Roger Mortimer, and granddaughter Anne Mortimer, who married a son of the 1st Duke of York.
Philippa died in 1382, and was buried at Wigmore Abbey, Herefordshire.
Her children with Edmund Mortimer were as follows:
|Lady Elizabeth Mortimer||12 February 1371||20 April 1417||She first married Sir Henry 'Hotspur' Percy, with whom she had two children, Henry Percy, 2nd Earl of Northumberland, and Lady Elizabeth Percy. Her second husband was Thomas de Camoys, Baron Camoys, with whom she had a son, Lord Roger de Camoys. Elizabeth Mortimer was an ancestor of the third Queen Consort of Henry VIII, Jane Seymour.|
|Roger Mortimer, 4th Earl of March||11 April 1374||20 July 1398||He married Lady Alianore Holland, by whom he had four children, Anne, Edmund Mortimer, 5th Earl of March, Eleanor, and Roger. The House of York's claim to the throne was through his eldest daughter, Anne Mortimer.|
|Lady Philippa Mortimer||21 November 1375||26 September 1400||She first married John Hastings, 3rd Earl of Pembroke. Her second husband was Richard Fitzalan, 11th Earl of Arundel, with whom she had a son, John, who died young. Her third husband was Sir Thomas Poynings of Basing, 5th Baron St. John.|
|Sir Edmund Mortimer||9 November 1376||13 May 1411||Married Catrin (Catherine) Glyndŵr, the daughter of Owain Glyndŵr. They had issue, possibly a son named Lionel, said to have died young, and three daughters who died in the Tower of London alongside their mother.|
And one other child, John de Mortimer (c. 1378-1424), Knight.
|Ancestors of Philippa, 5th Countess of Ulster|
Philippa, 5th Countess of UlsterBorn: 16 August 1355 Died: 5 January 1382
|Peerage of Ireland|
Elizabeth de Burgh
and Lionel of Antwerp
|Countess of Ulster
with Edmund Mortimer