2009-10: John Mitchell skated in 41 games for the Wisconsin Badgers as a senior in his final year of college hockey. He scored 8 goals with 11 assists and was plus-nine with 54 penalty minutes. Three of his eight goals were scored on the Badgers power play. Wisconsin was one of the top teams in college hockey – finishing second in WCHA and defeating Denver in the league's third place game and then reaching the Frozen Four championship game against Boston College.
2010-11: Mitchell signed an AHL contract with Anaheim affiliate Syracuse in his first pro season – skating in 60 games for the Crunch as well as spending three games with the ECHL's Elmira Jackals. Playing predominantly a third line, two-way forward role, he scored 9 goals with 19 assists and was minus-one with 64 penalty minutes. Mitchell was sent to Elmira briefly at the beginning of November and was minus-two with 1 goal and 1 assist for the Jackals.
2011-12: Mitchell re-signed with Syracuse for a second AHL season and skated in 50 games for the Crunch – missing time at the beginning of the year due to back spasms and then missing a handful of games in February. He scored 8 goals with 10 assists and was plus-seven with 23 penalty minutes during the regular season. Syracuse finished fourth in the East Division. Mitchell played in all four games in the first round playoff series with Saint John's, scoring 1 goal and finishing an even plus/minus. He signed a two-year, two-way free agent contract with the Ducks in May 2012.
Mitchell was dealt by the Anaheim Ducks to the Washington Capitals in the deal that brought Mathieu Perreault to the Ducks.