John Mordaunt, 1st Viscount Mordaunt (18 June 1626 – 5 June 1675) was an English royalist. He was born in Lowick, Northamptonshire, the second son of John Mordaunt, 1st Earl of Peterborough and Elizabeth Howard (d. 1671), daughter of William Howard, 3rd Baron Howard of Effingham. In June 1648, he joined his brother, Henry Mordaunt, 2nd Earl of Peterborough in leading a Royalist uprising, and fled with him to the Continent when it failed. He had returned to England by 1652, and married Elizabeth Carey on 7 May 1657.