He served as a deputy representative to the Parliament of Norway from Telemark during the term 1969–1973. He was a member of Skien city council from 1963 to 1971, and chaired the local Labour Party chapter from 1969 to 1971. In 1973 he was named Minister of the Environment in Bratteli's Second Cabinet. During a cabinet reshuffle in 1974 he became Minister of Social Affairs, first acting from April, then permanently from September.[1] He replaced the deceased Sonja Ludvigsen, and in August 1974 the newspaper Verdens Gang wrote, wrongly, that the "only thing which can be said to be certain now, is that the new Minister of Social Affairs will be a woman".[2] Halvorsen sat throughout the time of Bratteli's Second Cabinet, to 1976. In 1977 he became leader of the Confederation of Trade Unions, a position he kept until his death in 1987. At the same time he was a member of the Labour Party central board from 1977 to 1987.[1]