Hamilton also served in the Legislative Assembly of Manitoba from 1915 to 1920 as a member of the Liberal Party. He first sought election to the Manitoba legislature in the 1914 provincial election, and lost to Conservative Harry Mewhirter by 364 votes in the Elmwood constituency. He ran again in the 1915 election, and defeated new Conservative candidate D. Munro by 1,453 votes. The Liberals won a landslide majority in this election, and Hamilton served as a backbench supporter of Tobias Norris's government for the next five years.