Nicknamed "Big Baby", he played for the Boston Celtics, Orlando Magic, Los Angeles Clippers, and the St. John’s Edge.

After playing college basketball with the LSU Tigers, Davis was selected by the Seattle SuperSonics with the 35th overall pick in the 2007 NBA draft.

In the crucial game of the national semifinals, LSU lost to UCLA, trailing by a wide margin in the first half and never managing a comeback.

On March 20, 2007, Davis held a press conference to announce that he would forgo his junior season at LSU and enter his name into the 2007 NBA draft.

With the departures of Ryan Gomes, Gerald Green, and Al Jefferson, Davis was expecting to see a lot of playing time his rookie season.

Though he is primarily a power forward, Davis also spent some time playing as the team's backup center in the regular season.

[4] After coming off the bench for the first 19 games of the season, Davis made his first NBA start against the Sacramento Kings on December 12, 2007, in place of injured center Kendrick Perkins.

[5][6] Playing at power forward with Kevin Garnett taking Perkins' place at center, he scored 16 points and pulled down 9 rebounds as the Celtics won the game 90–78.

[7][8] His breakout performance took place against the Detroit Pistons on January 5, 2008, as he scored 16 of his 20 points in the fourth quarter, including the last basket of the game.

[14] On May 27, 2010, during the 2010 Eastern Conference Finals against the Orlando Magic in Game 5, Glen Davis suffered a severe concussion when he got elbowed in the face by Dwight Howard.

[20] Davis became an unrestricted free agent following the 2014–15 season and had left ankle surgery in September 2015,[21][22] sidelining him from basketball-related activities for eight to 12 weeks.

[28] In the summer of 2001, at only 15 years of age, Davis attended a basketball camp run by LSU alumnus and future Boston Celtic teammate Shaquille O'Neal on the campus of Louisiana State University.

The encounter left a strong impression on O'Neal; ultimately, the incident helped Davis to get introduced to former LSU head coach Dale Brown.