Called to the Bar in 1840, Brett went to the northern circuit and became a Queen's Counsel in 1861. On the death of Richard Cobden in 1865, he unsuccessfully contested Rochdale as a Conservative, but in an 1866 by-election, he was returned for Helston in unique circumstances. He and his opponent polled exactly the same number of votes, and the mayor, as returning officer, then gave his casting vote for the Liberal candidate. As the vote was given after four o'clock, however, an appeal was lodged, and the House of Commons allowed both members to take their seats.