Thomas Clifford was born on 25 March 1414, the elder son and heir of John, Lord de Clifford by Elizabeth Percy, daughter of Henry 'Hotspur' Percy and Elizabeth Mortimer, daughter of Edmund Mortimer, 3rd Earl of March.

He had a younger brother, Henry Clifford,[1] and two sisters, Mary and Blanche.

Clifford inherited the barony and the title of High Sheriff of Westmorland at the age of seven upon his father's death at the Siege of Meaux on 13 March 1422.

[4] He was succeeded by his elder son, John, 9th Baron de Clifford.

After March 1424 Clifford married Joan Dacre, the daughter of Thomas, 6th Baron Dacre of Gilsland, by Philippa, daughter of Ralph Neville, 1st Earl of Westmorland, by whom he had four sons and five daughters:[5] According to Shakespeare's, Henry VI, Part 3 following Hall's Chronicle and Holinshed's Chronicles, it was Thomas Clifford's son and heir, John Clifford, 9th Baron de Clifford, who slew, in cold blood after the Battle of Wakefield, the young Edmund, Earl of Rutland, son of Richard, 3rd Duke of York, cutting off his head and sending it crowned with paper to Henry VI's wife, Margaret of Anjou, although later authorities state that Lord Rutland had been slain during the battle.