2006-07: Garrett Thompson played for the Soo Indians Major Midget team along with his older Marc Thompson after skating for the Davison High School as a sophomore the previous season (Davison reached the Michigan state semifinals in 2005-06). Thompson scored 6 goals with 4 assists and 30 penalty minutes in 26 MWEHL games.
2007-08: Thompson joined the Traverse City North Stars of the North American Hockey League as a 17-year-old. In 24 games he scored 5 goals with 2 assists and was -5 with 27 penalty minutes. The North Stars finished fifth in the North Division.
2008-09: Thompson skated in 40 of 58 regular season games and all five playoff contests in his second season with Traverse City. He scored 11 goals with12 assists and was +12 with 26 penalty minutes as the North Stars finished third in the North Division. Thompson had 2 assists and was +4 with 2 penalty minutes in five playoff games.
2009-10: Thompson was the second-leading scorer for Traverse City in his third season with the North Stars. He led the team with 24 goals and had 26 assists — finishing +30 with 75 penalty minutes — in 57 regular season games. The North Stars finished first in the North Division and reached the playoff semifinals. Thompson scored 6 goals with 4 assists and was +1 with 2 penalty minutes in ten playoff games. In April he committed to playing college hockey at Ferris State in 2010-11.
2010-11: Thompson appeared in 18 games for Ferris State in his freshman season. He scored 4 goals with 2 assists and had an even plus/minus with 2 penalty minutes. The Bulldogs finished fifth in the CCHA and lost to Western Michigan in a three-game CCHA quarterfinal series.
2011-12: Thompson skated in 41 of 43 games in his sophomore season as Ferris State reached the NCAA Frozen Four championship game against Boston College. He scored 11 goals with 12 assists and was an even plus/minus with 20 penalty minutes. The Bulldogs finished first in the CCHA during the regular season but were upset by Bowling Green in the conference quarterfinals. Ferris State defeated Denver and Cornell in back-to-back 2-1 games to capture the NCAA Midwest Regional and defeated Union, 3-1, to reach the championship game. Thompson scored the lone goal for the Bulldogs in a 4-1 loss to BC.
2012-13: Thompson attended the Calgary Flames prospect camp in the summer before returning to Ferris State for his junior season. He was the third-leading scorer for the Bulldogs with 11 goals and 15 assists and was +8 with 22 penalty minutes in 37 games. Ferris State finished fifth during the regular season and lost to Ohio State in a three-game quarterfinal series in the final season of the CCHA.
2013-14: Thompson returned to Ferris State for his senior season after participating in the Chicago Blackhawks prospect camp during the summer. One of two Bulldogs to skate in all 43 games, he shared the team scoring lead with Cory Kane and Justin Buzzeo. Thompson led the Bulldogs with 16 goals and had 16 assists while finishing +12 with 42 penalty minutes. Ferris State finished first in the regular season and reached the Broadmoor Trophy Championship game against Minnesota State in its first season in the WCHA. After losing 4-1 to Minnesota State the Bulldogs received an at-large bid to the NCAA Tournament Midwest Regional; defeating Colgate 1-0 before falling to North Dakota 2-1 in double overtime. Ottawa signed Thompson to a one-year entry-level contract on April 1st, 2014.
2014-15: Thompson skated in 65 games for Ottawa AHL affiliate Binghamton in his first pro season. He scored 6 goals with 8 assists and was -13 with 33 penalty minutes. Binghamton missed the AHL playoffs, finishing third in the East Division.
Thompson was not tendered a qualifying by the Ottawa Senators following the 2014-15 season and is currently an unrestricted free agent.