[3] He received the nickname "Gumby" in college from a teammate, describing Montgomery's long limbs and lack of coordination.

[11] Montgomery received a non-roster invitation to spring training in 2017,[12] and competed for a job in their Opening Day starting rotation.

[14][15] In his major league debut against the Tampa Bay Rays, Montgomery received a no-decision as he allowed three runs (two earned) in 4+2⁄3 innings, striking out seven.

[20] On July 25, against the Cincinnati Reds, Montgomery took a no-hitter into the sixth inning before Scott Schebler hit a double to break up the bid.

[23] On August 12, Montgomery was struck in the head by an errant foul ball during batting practice while signing autographs prior to the game;[24] he was not seriously hurt.

[27] However, on June 5, it was announced that Montgomery tore his ulnar collateral ligament while throwing on flat ground during rehab a few days prior and would undergo Tommy John surgery, ending his season.

[32][33] The Yankees traded Montgomery to the St. Louis Cardinals for Harrison Bader and a player to be named later or cash considerations on August 2, 2022.

[35] On August 22, 2022, Montgomery pitched his first complete game in the major leagues, a one-hit, 99-pitch Maddux against the Chicago Cubs.

[37] On July 30, 2023, the Cardinals traded Montgomery and Chris Stratton to the Texas Rangers in exchange for John King, Tekoah Roby, and Thomas Saggese.

[43] The Rangers made the 2023 MLB postseason and Montgomery started Game 1 of the 2023 American League Wild Card Series.

[45] Montgomery also started Game Two of the 2023 American League Division Series; he pitched four innings and did not receive a decision in the Rangers' 11-8 victory over the Baltimore Orioles.

[47] Montgomery also started Game Five of the 2023 ALCS; he hurled 5 1/3 innings, gave up two earned runs, and did not receive a decision in the Rangers' 5-4 win.

[55] Montgomery was recalled on April 19 and later made his Diamondback debut that same day against the San Francisco Giants, allowed just one run over six innings.

[57] However, following 19 starts where he accumulated a 6.44 ERA over 95 innings pitched, the Diamondbacks announced on August 23 that Montgomery would move to the bullpen.