Speaker of the House of Commons through the years of the Revestment, and the friendliest ear the Manx delegation found in London.

He was returned for Grantham and chosen Speaker in 1761, selected for the chair by Lord Bute on account of his Tory politics. One account describes him as a country gentleman of good family and large estates and of an amiable disposition. Horace Walpole allowed that, bating his nose, the chair seemed to be well filled. He was re-elected Speaker in 1768, resigned in 1770, and was succeeded by Sir Fletcher Norton, whom the Duke of Grafton pressed to take the chair on Cust's death.

When George Moore came over from Peel on the Keys' business, Cust was the one man among all those he approached who was warm in his sympathy. He showed great kindness and consideration, advised on the form and matter of the memorial, and was afterwards thanked by the Keys. He was also honest about the prospects. When Chatham reappeared in London in March, having been absent and the business delayed, both the Duke of Atholl and Cust told Moore that all hope of anything being done that session was gone, and Moore went home.

In the chair he was a stickler for form. When Grenville moved to amend a resolution so that troops kept up in America should be paid by America, Cust told the House he thought himself obliged in point of duty to say he doubted whether the amendment could be received consistently with its rules, citing the resolution of 18 February 1667 on motions for charges upon the people.