[1] He made his professional debut with the rookie-level GCL Twins, and recorded a 3.27 earned run average in 6 games.

In 2010, Tonkin split the year between the rookie-level Elizabethton Twins and the Class-A Beloit Snappers, pitching to a 4-6 record and 3.40 earned run average in 23 appearances between the two teams.

In 2012, Tonkin split the season between Beloit and the High-A Fort Myers Miracle, logging a cumulative 4-1 record and 2.08 earned run average in 44 appearances.

[3] He was assigned to the Double-A New Britain Rock Cats to begin the 2013 season, and received a promotion to the Triple-A Rochester Red Wings later in the year.

He ended the year having pitched in 16 games for the Twins, posting a 5.14 earned run average with 24 strikeouts in 21 innings.

On November 9, 2017, Tonkin's contract was sold to the Hokkaido Nippon-Ham Fighters of Nippon Professional Baseball (NPB).

[8] Tonkin registered a 3-0 record and 4.26 earned run average in 14 games with the Triple-A San Antonio Missions before he was released by the team on June 10.

On June 18, 2019, Tonkin signed with the Long Island Ducks of the independent Atlantic League of Professional Baseball.

On August 18, 2019, Tonkin's contract was purchased by the Arizona Diamondbacks organization and he was assigned to the Triple-A Reno Aces.

[10] On March 31, 2021, Tonkin signed with the Long Island Ducks of the Atlantic League of Professional Baseball.

[13] He made 47 relief appearances for the Triple–A Gwinnett Stripers, logging a 3.17 earned run average with 73 strikeouts and 16 saves in 48+1⁄3 innings pitched.

[14] Pitching in relief of Kyle Wright on April 11, 2023, Tonkin earned his first major league victory since 2016.