Sabo was born of Norwegian immigrant parents in Crosby, North Dakota and in 1959 received a B.A. from Augsburg College in Minneapolis, later pursuing graduate studies at the University of Minnesota. He was elected to the Minnesota House of Representatives in 1960 at the age of 22, later serving as minority leader (1969-72) and as house speaker (1973-78). During his tenure in the state house he served terms as president of the National Conference of State Legislatures and of the National Legislative Conference, and was a presidential appointee to the National Advisory Commission on Intergovernmental Relations.

When Congressman Don Fraser stepped down to run for the US Senate, Sabo became the Democratic candidate to succeed him. He won easily in November 1978 and has been reelected thirteen times without anything resembling serious opposition.

During the 103rd Congress (1993-94) he chaired the House Budget Committee. In the present (109th) Congress he sits on the House Appropriations Committee, and is the ranking member of that committee's Homeland Security subcommittee.

A Lutheran, Sabo is married and has two children and six grandchildren. His daughter, Julie Sabo, is a former member of the Minnesota Senate and was the 2002 Democratic-Farmer-Labor Party nominee for Lieutenant Governor. During the course of his career Sabo referred to himself as a "liberal decentrist", preferring progressive politics, but local control instead of federal control.

On March 18, 2006 he announced that he will not run for reelection, ending 46 years as an elected official, including 28 years in Congress - the second-longest tenure in either house of Congress in the state's history, behind only fellow Democrat Jim Oberstar. The 5th is so heavily Democratic that whoever wins the DFL nomination is all but assured of being the district's next congressman - and only the fourth person to represent the district since 1943. Within a week of Sabo's announcement, there were a dozen candidates vying for the DFL endorsement.