Career
Tanner was first elected to the Manitoba legislature in the provincial election of 1920, defeating Liberal and Farmer candidates in the Winnipeg-area constituency of Kildonan and Saint Andrews. He was re-elected in the 1922 election by a greater margin. Tanner served on the opposition benches throughout his time in the legislature.
He appears to have sought re-election in the 1927 campaign as a supporter of the Progressive government of John Bracken, though still identifying himself as a "Labour" candidate.
He was defeated, finishing third against Conservative James McLenaghen. Tanner ran for the Canadian House of Commons in the federal election of 1930, as a candidate of the Independent Labour Party in Winnipeg South.
He finished a distant third against Conservative Ronald Rogers. At the time, he listed his occupation as "editor".