[1] In 2012, he posted a combined record of 5–12 in 27 starts while splitting the season between the Double-A Northwest Arkansas Naturals and Triple-A Omaha Storm Chasers.

[2] On December 9, 2012, Montgomery was traded to the Tampa Bay Rays along with Jake Odorizzi, Patrick Leonard, and Wil Myers for pitchers James Shields and Wade Davis.

[5] After an injury sidelined starter James Paxton, the Mariners called Montgomery up to the majors for the first time on June 2, 2015.

He started that night against the New York Yankees at Safeco Field, giving up one run, four hits, and walking two while striking out four and leaving after six innings with a 2–1 lead.

[6] On June 23, Montgomery became the first Mariners left-handed pitcher to ever throw a complete game shutout with 10 strikeouts and no walks, getting the win against the Kansas City Royals 7–0.

[12] After allowing a three-run home run to the first batter he faced as a Cub,[13] Montgomery performed well during the remainder of the season.

[16] Montgomery earned his first professional save, helping the Cubs win the World Series for the first time in 108 years.

[1] On July 15, 2019, Montgomery was traded back to the Royals, with the Cubs receiving catcher Martín Maldonado.

[20] Despite Montgomery being primarily a reliever and spot starter during his major league career, the Royals used him exclusively as a starting pitcher.

[23] On February 14, 2021, Montgomery signed a minor league contract with the New York Mets with an invitation to spring training.

In response to his ejection, Montgomery struck an umpire with a rosin bag, and threw his uniform on the field of play before retreating to the clubhouse.

[30] He made 22 appearances (17 starts) for the Triple-A Syracuse Mets, struggling to a 2–10 record and 6.72 ERA with 54 strikeouts in 69+2⁄3 innings pitched.