MacKinnon attended Oliver Ames High School in Easton, Massachusetts, where he played baseball and soccer.

[7][8] He opened the 2022 season with the Salt Lake Bees and earned Pacific Coast League Player of the Month honors in May.

[12] He recorded his first major league hit on June 22, an RBI single in the seventh inning against the Kansas City Royals.

[15] He was assigned to the Triple-A Las Vegas Aviators of the Pacific Coast League and initially spent eight games with the team.

On December 19, 2022, MacKinnon signed a one-year, 90 million yen ($684,000) contract with the Saitama Seibu Lions of Nippon Professional Baseball (NPB).

[20] On April 2, MacKinnon hit his first NPB home run, a solo shot off Taisuke Yamaoka of the Orix Buffaloes.