Seweriin had been a member of Oslo city council from 1937 until the war broke out, and also served briefly in 1945. She was the deputy leader of the Workers' Youth League from 1937 to 1946. After the war she was a member of the Labour Party secretariat for women from 1945 to 1971—from 1953 to 1963 she chaired the secretariat, and was a member of the Labour Party's central committee. She was elected as a member of the Parliament of Norway in 1945 from the constituency Oslo, and was re-elected in 1949, 1953, 1957, 1961 and 1965. On 2 November 1953, she temporarily stepped out of parliament as she became Minister of Social Affairs in Torp's Cabinet. She had worked with social policy as a parliamentarian, but in an interview with Trond Nordby she confessed to having lacked a clear purpose with her position as Minister of Social Affairs. Legislative work in the ministry slowed, and as leader of the ministry she was clearly dominated by the bureaucrat staff, especially Karl Evang. She continued as Minister of Social Affairs until 1 August 1955, a few months after the formation of Gerhardsen's Third Cabinet. During this time, her seat in parliament was filled by Hjalmar Larsen, Aase Lionæs and Gunnar Alf Larsen. When she stepped down in 1969, her time of 24 years as a parliament member was a record for a woman in Norway.