After graduating from Beaver Dam High School, he enlisted in the United States Army for duty in World War II.

[2] After returning to the United States, he continued his education at Carleton College, in Northfield, Minnesota, and at the University of Wisconsin Law School, where he received his LL.B.

Kastenmeier made an unsuccessful bid for Congress in Wisconsin's 2nd congressional district in 1956, losing to Republican Donald Tewes by 11 points.

However, he romped to a fourth term in 1964 and was re-elected with comfortable majorities twelve more times over the next 24 years, serving from the 86th United States Congress to the 101st.

Kastenmeier helped to frame the impeachment resolution against Claiborne which ultimately resulted in his conviction in the United States Senate and removal from office.

[8] A week before his death in 2015, in an interview with Madison's Capital Times, Kastenmeier warned of American overseas interventions, saying, "There are far too many trouble spots in the world.