McMahon attended Mater Dei High School in Santa Ana, California where he played baseball and was a quarterback on his football team.

In 2014, he played for the Asheville Tourists where he posted a .282 batting average with 18 home runs, 102 RBIs, and 46 doubles in 126 games, and in 2015, he played with the Modesto Nuts where he slashed .300/.372/.520 with 18 home runs, 75 RBIs, and 43 doubles in 132 games.

McMahon spent 2016 with the Hartford Yard Goats where he batted .242 with 12 home runs and 75 RBIs in 133 games.

[6] The Rockies promoted McMahon to the major leagues on August 11, 2017, primarily to play first base.

It was McMahon's first career walk-off homerun and came only a day after he had hit another go-ahead home run against the Dodgers that proved to be the game winner.

On March 21, 2022, McMahon signed a six year, $70 million contract extension with the Rockies.