Siegfried played college basketball for Ohio State University, and his tenure there overlapped with future Hall-of-FamersJerry Lucas and John Havlicek. Siegfried, a junior high scoring guard, and Joe Roberts, a senior forward, were the two holdover starters when three outstanding sophomores, Lucas, Havlicek and guard Mel Nowell arrived for the 1959–60 season. Siegfried adjusted his scoring to allow for Lucas and Nowell while joining Roberts and Havlicek as a key defender. Siegfried was also an excellent free throw shooter few risked fouling. The Ohio State Co-Captain of the 1960 team, Siegfried played a key role in the Buckeyes run to the 1960 NCAA title. All five starters from that team later played in the NBA, which then had just nine teams and eleven players per team. Future coach Bobby Knight was a reserve on that team as well. Said Knight of Siegfried, "I never saw a better guard in the Big Ten than Larry Siegfried. He was a great player. He was tough as hell. He was physical, he could jump . . . if I had my choice of any guard who played in the Big Ten when I coached and everything else, I'd have a hard time picking someone else."
For the 1960–61 season, Siegfried was team captain outright. The team went undefeated until the NCAA Final, when they were upset by Cincinnati. Siegfried was named to the NCAA Final Four All-Tournament Team. Also named All-Big Ten, Siegfried did not get the All-American consideration he may have been due because of the star presence of Lucas. Siegfried did play in the 1960 US Olympic Trials for the Rome Games.
At 6'3" and 190 pounds, Siegfried was considered a prototype guard for the NBA at that time. The Cincinnati Royals drafted him with their first pick in 1961 to pair with Oscar Robertson in their backcourt. Siegfried would not play in Cincinnati because of Ohio State's loss to Cincinnati's Bearcats that year. Instead, he joined the Cleveland Pipers of the American Basketball League. The team, owned by future Yankee boss George Steinbrenner, and coached by John McLendon and Bill Sharman, won that pro league's 1961–62 title. Dick Barnett and Connie Dierking were among that team's stars. The highly drafted Siegfried was just a reserve.
With perennial champion Boston Celtics (1963–70)
When the ABL folded the next year, the St. Louis Hawks acquired his rights but then surprisingly cut him. Siegfried considered retirement, becoming a high school coach and teacher before former college teammate Havlicek convinced coach Red Auerbach to try him out for the Boston Celtics. Slowly regaining his confidence, Siegfried proved to be a key pickup. He eventually became a starter next to Havlicek or Sam Jones in the backcourt. His defense and free throw shooting were key to NBA title wins for Boston in 1968 and 1969. Boston announcer Johnny Most often noted his tenacious defense, calling 'Ziggy's in his shirt tonight' to describe Siegfried on many nights.
Siegfried played his first seven professional seasons with the Boston Celtics, earning five championship rings during that time. He led the NBA in free throw percentage in both the 1965–66 and 1968–69 seasons.