A follower of Lord Howe, Brett was Paymaster of the Navy from 1766 to 1770. He entered Parliament in 1768 as member of parliament for Lostwithiel. He resigned that seat in 1776, to be returned instead as the government-sponsored MP for Sandwich, a constituency with a strong naval connection. From 1777 he voted consistently with the opposition, and in 1780 lost his seat when defeated by two government-backed candidates.
