In 1970, Johnson was elected to the
Wisconsin State Assembly in
Rock County's 3rd district, defeating incumbent Republican
George B. Belting. He was subsequently reelected 7 times, serving until 1985. In 1980, Democratic Assembly Majority Leader
James W. Wahner announced he would resign from the Assembly in order to accept an appointment in the federal government. Johnson, considered a moderate, was elected the new majority leader on the 2nd ballot, defeating
Thomas A. Loftus (of the liberal wing) and
Thomas A. Hauke (conservative). He remained majority leader until leaving the Assembly in 1985. After leaving the Assembly, he worked briefly as a lobbyist for
Wisconsin Bell, but, in 1986, accepted appointment as Assistant Superintendent in the
Wisconsin Department of Public Instruction, under Superintendent
Herbert J. Grover. He left government service again in 1990, and worked as a lobbyist until retiring in 1996. ==Personal life==