In 1936 he was elected to the Wisconsin State Assembly on the ticket of the Wisconsin Progressive Party (the Socialists and Progressives were practicing a form of electoral fusion during this period; but his biography clearly identified him as an active Socialist) from the 2nd Milwaukee County district (the 2nd and 10th Wards of the City of Milwaukee), unseating Democratic incumbent Clarence Kretlow, with 6,767 votes to 5867 for Kretlow and 2129 for Republican Alex Klose.

[3] He was re-elected once more in 1940, with 7,812 votes to 4,869 for former Socialist State Representative Otto Kehrein (now running as a Republican) and 4095 for Democrat Elmer Foerster.

[4] Biemiller continued to work as a special organizer for the Wisconsin State Federation of Labor through 1942, when he moved to Washington to take a position in the War Production Board.

In 1944 he was elected as a Democrat (Biemiller had abandoned both the Socialist Party and the Progressives by then) to the 79th Congress from the Milwaukee-based Wisconsin's 5th congressional district, unseating Republican incumbent Lewis D. Thill, with 88,606 votes to Thill's 78,834, Socialist former State Representative Edwin Knappe's 4,758, and 2,103 for Independent Progressive Irwin Aaron.

During this period, he played a significant role in passing civil rights, Medicare, and other social and economic legislation, especially during the Kennedy and Johnson administrations.