In 1936 he was elected to the Wisconsin State Assembly on the ticket of the Wisconsin Progressive Party from the 2ndMilwaukee County district, unseating Democratic incumbent Clarence Kretlow, with 6,767 votes to 5867 for Kretlow and 2129 for Republican Alex Klose. He was appointed to the standing committees on the judiciary and public welfare, to a specialjoint committee on "Legislation on Administration of State Government", and perhaps most importantly to the "Interim Committee" appointed in 1937 by the Legislature to discuss reorganization of Wisconsin government. He was re-elected in 1938, with 5,098 votes to 2934 for Republican Edward J. Mueller and 2848 for Democrat William W. Murphy. He remained on the judiciary committee, and was appointed to special committees on "Revenue Needs of the State for the Current Biennium" and automobile title laws. He was re-elected once more in 1940, with 7,812 votes to 4,869 for former Socialist State RepresentativeOtto Kehrein and 4095 for Democrat Elmer Foerster. He was moved to the committee on engrossed bills and to an additional special committee on "Subversive and Un-American Activities of Certain Groups of Employes Engaged in the Manufacture of National Defense Materials". His official biography no longer listed him as an active member of the Socialist Party, but did describe him as a member of the Progressive Party Federation; and he became the floor leader of the Progressive Party in the Assembly. 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. He did not run for re-election, and was succeeded in the Assembly by Democrat Michael F. O'Connell.
Congress
In 1944 he was elected as a Democrat 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. He was defeated for re-election in 1946 by Republican Charles J. Kersten, and went to work as director of political education for the Upholsterers Union. During this period Biemiller joined many other former Socialists and Progressives in helping to found Americans for Democratic Action. He cooperated with Hubert Humphrey in successfully calling for a strong civil rights plank at the 1948 Democratic National Convention, to which he was a delegate and chair of the platform committee which produced the plank. He defeated Kersten in a 1948 re-match for election to the 81st Congress, but once more lost a bid for re-election in 1950 to Kersten.
After Congress
After losing his seat in Congress, Biemiller worked through the remainder of the Truman administration at the Department of the Interior. He served from 1953-1956 as a legislative representative for the American Federation of Labor, and then Director of the Department of Legislation of the AFL-CIO from 1956–1978. 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.