[1] 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.

However, although trusted by the King, Ormonde, and Hyde, many royalists (including the members of the Sealed Knot) disliked and mistrusted him.

However, the new rising he planned in June was several times delayed, partly because of friction with other royalists, and the Council of State ordered his arrest.

The rising, in Surrey, drew only thirty followers, and Mordaunt narrowly escaped to France in September.

He returned to England again in October after the expulsion of the Rump Parliament, and was planning another rising, but his connections with Hyde occasioned suspicion and he received little favour from any party.