Hay resigned following a disastrous election result, and Sinclair served as interim leader of the Ontario Liberal Party from 1923 to 1930 and also as Leader of the Opposition in the provincial legislature.
He was interim leader for that length of time (and through the elections of 1926 and 1929) because of the party's state of disorganization and inability to hold a proper leadership convention.
Sinclair initially decided to run for the permanent leadership in 1930, but because of the party's failure to make gains during his tenure, he attracted little support, and withdrew before balloting began.
Sinclair was re-elected in the 1934 election that brought the Liberals to power but Hepburn, the new Premier of Ontario, did not appoint him to Cabinet.
Sinclair sat as a government backbencher for three years and then retired from the legislature at the 1937 provincial election.