Mahrley made his major league debut on August 3, 2017 for a game between the New York Mets and the Colorado Rockies at Coors Field.

He was at third base, with Lance Barrett at second, Brian Knight at first, and Jim Reynolds as home plate umpire.

[4] MLB promoted Mahrley to the permanent staff in 2023 after a record-high ten umpires retired following the 2022 season.

[5][2] His first MLB postseason assignment was the 2024 American League Division Series between the Cleveland Guardians and the Detroit Tigers.

[6] Oh August 25, 2024, Mahrley had to be carted off the field at Yankee Stadium after being hit on the side of his neck by the barrel of a broken baseball bat from Giancarlo Stanton while he was behind home plate.