Tonnemaker, weighing in at 11 pounds, was born on June 8, 1928 on a farm near Ogilvie, MN, to Anna Nelson and Frank Clayton Tonnemaker. After his father died when Clayton was 7, he and his mother and sister, Lucille, sold their farm at auction and moved to the town of Rush City, MN. The family later moved to Northeast Minneapolis, and Clayton attended Edison High School.
Tonnemaker lettered in football at Rush City High School as an 8th grader. After moving to Minneapolis, Tonnemaker played center for the Edison football team, serving as captain and winning All-City Honors. He unofficially played for the Minnesota Gophers while in high school, even scoring a touchdown during a 1946 spring season scrimmage. It was not legal for a high schooler to train with a college team at the time, so the Gophers didn’t acknowledge this.
Tonnemaker officially began playing center linebacker for the Gophers during his freshman year, 1946, when a World War II-era ruling made it legal for freshman to play in the Big Ten. Before the war this was not allowed. He became part of a group of Gopher players known as the '49ers, their year of graduation. He was a regular from mid-freshman year, with the Gophers winning 23 out of 30 games, and a “win-loss edge over every Big Ten rival except Michigan”. Along with Leo Nomellini, Tonnemaker was part of a defensive line that allowed “an average of less than nine points a game in the '49ers’ final season”.
Originally drafted by the San Francisco 49ers, his pro contract was transferred to the Green Bay Packers after the American Conference merged with the NFL in 1950 and the rules changed. The Packers made him their number one National Football League draft pick in 1950 (4th in the NFL overall), and paid him $8,000 a year, the top salary on the team. Tonnemaker played center and middle linebacker.
Named All-Pro in his rookie season (1950) and again after his war service in 1953
Picked to play in the first Pro Bowl ever (January 14, 1951) but missed it because he had to report for Army duty