He played with the National Basketball Association's (NBA) New York Knicks from 1956 to 1963 and was a player-coach of the St. Louis/Atlanta Hawks franchise where he spent nine years.

While a reservist, Guerin attended Iona College from 1950 to 1954 where he scored 1,375 points in 67 games playing for coach Jim McDermott.

After graduation, Guerin served on active duty at Marine Corps Schools, Quantico, Virginia for two years.

As a member of the New York Knicks, his feisty on-court style and wisecracking off-court demeanor catered to Madison Square Garden crowds.

He was traded to the St. Louis Hawks midway through the 1963–64 season and spent the next eight years as the team's player-coach and then head coach.

With St. Louis (and eventually Atlanta), Guerin played alongside Bob Pettit, Lou Hudson, Lenny Wilkens, and Cliff Hagan.

Guerin helped the Hawks to nine consecutive playoff appearances and was named NBA Coach of the Year for 1967–68.

He averaged at least 19.9 points per game in his three years of varsity basketball, and set the freshman team scoring record at the time.

The 21 assists he totaled were also Madison Square Garden high until John Stockton broke the record 41 years later.

[9] That year New York made its only postseason appearance with Guerin on the team, losing to the Syracuse Nationals in a first-round sweep.

On October 18, 1963, two games into the 1963–64 season, the Knicks traded Guerin to the St. Louis Hawks for cash and a second-round draft pick (Howard Komives was later selected).

[8] In his first appearance at the Garden in a Hawks uniform, Knicks fans showed their gratitude by giving Guerin a five-minute standing ovation.

[4] Guerin joined a Hawks team loaded with offensive weapons, and his production dropped accordingly to 13.1 points per game in 1963–64.

The Hawks earned a playoff spot but lost to the Baltimore Bullets in a division semifinal series.

Feeling the need for a promoter as general manager, the Hawks replaced Guerin with Pat Williams two days later on August 6.

He served on active duty with the T&T Regtiment, Marine Corps Schools, Quantico, Virginia until his discharge as a first lieutenant in June 1956.