2005-06: Garrett Mitchell played for the Regina Pat Canadians in the Saskatchewan midget hockey league and was drafted by the Regina Pats in the second round (33rd overall) of the 2006 WHL Bantam Draft.
2006-07: Mitchell made his WHL debut appearing in four games with the Pats and notching one assist and was -2 with 2 PIM. Mitchell played the bulk of the season with the Pat Canadians in the SJHL and had 1 assist with 4 PIM in four playoff games.
2007-08: Mitchell established himself as a heavy-hitter in his rookie WHL season. In 62 games, he scored 8 goals with 5 assists and was +4 with 73 PIM. Mitchell had 1 goal with 6 PIM and was -3 in 6 playoff games as the Pats fell to Swift Current in the first round. Mitchell played for Canada West at the World U-17 Hockey Challenge and scored 4 goals with 2 assists and 20 PIM in six games.
2008-09: Mitchell was a physical force for Regina in his second season. Mitchell finished with a team-leading 140 PIM while scoring 10 goals with 5 assists; and was -15 in 71 games for the Pats. Regina finished fifth in the East division, winning just 27 games, and missed the WHL playoffs. Mitchell represented Canada at the WJC U-18 tournament and had 1 assist with 6 PIM in a checking line role. He was part of Team Canada for the Ivan Hlinka tournament. Mitchell was ranked 148th among North American skaters in the 2009 NHL Draft by Central Scouting.
2009-10: Mitchell despite lacking big offensive numbers and playing on a last-place Regina squad impressed Washington enough to receive a look-see with AHL Hershey following the WHL season and was tendered a qualifying offer in June to allow the Capitals to retain his rights. Mitchell made his AHL debut and was scoreless with two shots on goal in the Bears' 3-1 loss to Syracuse on April 6. Mitchell was limited to 57 WHL games after suffering concussion-like symptoms in two separate instances, and scored 15 goals with 16 assists and was -15 with 140 PIM for the Pats.
2010-11: Mitchell made his pro debut with AHL Hershey after captaining the Regina Pats in his fourth WHL season. Mitchell signed a three-year, entry-level contract with the Capitals in March 2011 and in his two games with the Bears he was scoreless and -1 with 5 PMs. In 70 games with Regina, a team that missed the WHL playoffs, Mitchell tied for the team lead with 140 penalty minutes and was -11. He was the team's third-leading scorer with 18 goals and 34 assists.
2011-12: Mitchell played 65 games for Capitals AHL affiliate Hershey in his first pro season. Playing a hard, two-way game for the Bears, he scored 6 goals with 9 assists and was minus-four with 85 penalty minutes. Hershey reached the playoffs after finishing second in the East Division. Mitchell scored 1 goal and was an even plus/minus with no penalty minutes in the five-game playoff series with Scranton/Wilkes-Barre. He played two ECHL games with South Carolina and had no points with 7 penalty minutes.
Mitchell is an energy player with third-line checking potential. The strength of his game is his determination and combativeness which he complements with above average speed and skating ability. The majority of his offense is generated through physical play and a willingness to compete in small areas as well as by driving to the net. Mitchell's offensive ability is limited due to either unwillingness or an inability to stick handle and control the puck - particularly when pressured. Mitchell prefers to angle opponents and use his body while putting the puck into safe areas and avoiding turnovers. He is willing to do the "dirty work" like penalty killing and working along the walls and he is solid as a shot blocker. Mitchell is not afraid to drop the gloves to lift his team's emotional level or to come to the aid of a teammate. He has strong leadership skills.