Leathes’ inheritance included an estate near Harwich, where he intended to stand at the 1727 British general election, but in the event was returned as Member of Parliament for Sudbury. In 1733 he was described as ‘a friend to the Government, a sure one, and has not given a vote against us’. In 1733 Leathes applied to Walpole for the government interest at Harwich for the next election, and Walpole agreed that this should go to Leathes and Lord Perceval, standing jointly. At the 1734 British general election, Leathes said he could not persuade his supporters to go along with the compact, and he joined with Charles Stanhope, against Perceval. Leathes became Recorder of Harwich in 1734.
At the 1741 British general election he nominated his brother, Hill Mussenden, to replace him at Harwich, and went back to Sudbury. He supported the Administration until the end of 1743, when he and his brother voted with the Opposition on a motion to discontinue the service of the Hanoverians from British pay. They abstained from both the two subsequent divisions on the Hanoverians in 1744 and 1746. He lost control of Harwich at the 1747 British general election when his brother was replaced there by a government nominee.