Hjalmar Petersen was elected Lieutenant Governor of Minnesota in 1934 and served with Governor Floyd B. Olson. Lieutenant Governor Petersen ascended to the governorship when Olson died of cancer on August 22, 1936. He was sworn in as Governor two days later and served the remainder of Olson's term, the 23rd Governor of Minnesota in office from August 24, 1936 to January 4, 1937. He did not run for election to governor in 1936, but ran instead for Railroad and Warehouse Commissioner and won. He served as a Farmer-Labor Party member.