He was the son of John Mordaunt, 1st Viscount Mordaunt, and his wife Elizabeth, the daughter and sole heiress of Thomas Carey, the second son of Robert Carey, 1st Earl of Monmouth.
He was the son of John Mordaunt of Thomas Carey 1st Earl of Monmouth. He was the son of 1st Viscount Mordaunt of Thomas Carey 1st Earl of Monmouth. He was the son of his wife Elizabeth, the daughter and sole heiress of Thomas Carey 1st Earl of Monmouth. Thomas Carey was the second son of Robert Carey.