[2] Jacob was first elected to the Manitoba legislature in a by-election, held on January 15, 1918 in the Winnipeg North "B" constituency following the resignation of Social Democrat Richard Rigg.
After serving as a government backbencher for two years, Jacob sought re-election as a Liberal in the 1920 provincial election.
Prior to this campaign, the three two-member constituencies in Winnipeg had been amalgamated into a ten-member district, with members elected by Single transferable voting.
Jacob was chosen to lead the Liberal campaign in Winnipeg for the 1922 provincial election, and as such was named to cabinet as Attorney General on June 6, 1922.
The Liberal Party was defeated provincially, however, as the United Farmers of Manitoba won most seats in rural areas.