Mary of Lancaster, Baroness Percy (c. 1320–1321 – 1 September 1362), was the youngest surviving child of Henry, 3rd Earl of Lancaster by his wife Maud Chaworth.
Through her father, she was a great-granddaughter of Henry III of England.
[1] She was a sister of Henry of Grosmont, 1st Duke of Lancaster, Maud, Countess of Ulster, Joan, Baroness de Mowbray, and Eleanor, Countess of Arundel.
Through her brother Henry, she was the aunt of Blanche of Lancaster, first wife of John of Gaunt and mother of Henry IV of England.
[3] They had the following surviving issue: Mary died at Alnwick, Northumberland, England on 1 September 1362, where she was also buried.