Montero signed with the New York Mets as an international free agent on January 20, 2011. Nearing his 21st birthday he signed at a much older age than most Dominican players. In his first professional season he went 5-4 with a 2.15 ERA and 66 strikeouts in 71 innings. In 2012, Montero went 11-5 with a 2.36 ERA, with 110 strikeouts and 19 walks in 122 innings. Prior to the 2013 season, Baseball America ranked him as the Mets' fifth best prospect. He started the season with the Double-ABinghamton Mets. He was promoted to Triple-A Las Vegas 51s in June. In 2013, with Binghamton and Las Vegas went 12-7 with a 2.43 ERA with Binghamton and a 3.05 ERA with Las Vegas, with 150 strikeouts and 35 walks in 154.4 innings pitched.
Major leagues
New York Mets
Montero was promoted on May 14, 2014, with Dillon Gee going to the disabled list with a lat strain. Montero started against the New York Yankees at Citi Field on the same day. In the top of the third inning, he struck out shortstop Derek Jeter for his first major league strikeout on a 3-2 fastball. He earned his second strikeout against Brett Gardner in the top of the fifth inning. Montero earned the loss as the Mets lost 0-4 as he pitched six innings giving up 5 hits, 3 runs, 2 home runs, 2 walks and struck out 3. He was sent down on May 31 to make room for Buddy Carlyle on the roster. On August 11, pitcher Jacob deGrom went on the disabled list with rotator cuff tendinitis. Montero was called up on August 12 in deGrom's place. On August 23, Montero was re-sent back to the 51s to make room for deGrom coming off the disabled list. He was recalled on September 6 as part of the September call-ups. He was later moved to the bullpen on September 13. Montero finished the season with a 1-3 record, 4.06 ERA in 10 games with 42 strikeouts in 44.1 innings pitched with a WHIP of 1.511 while giving up 44 hits, 21 runs, 8 home runs, and 23 walks. On April 17, 2015, he was sent down to the 51s to make room for Danny Muno. He was recalled up on April 27 to replace Muno on the roster. A day later he was once again sent down as Jack Leathersich was called up. On April 29, it was discovered that his rotator cuff was swelling and placed on the disabled list. He was placed on the 60-day disabled list on July 24. Montero would never pitch again in 2015. Montero finished the season with a 0-1 record, 4.50 ERA in 5 games with 13 strikeouts in 10 innings pitched with a WHIP of 1.400 while giving up 9 hits, 6 runs, and 5 walks. Montero began the 2016 season with the 51s. He was called up on April 12, with Eric Campbell being demoted to the 51s. Montero made one start with an ERA of 1.80 pitching five innings while giving up 3 hits, 2 runs, 2 walks and 5 strikeouts. Montero underwent Tommy John Surgery after suffering a UCL tear and missed the entire 2018 season. He elected free agency on November 3, 2018.
Texas Rangers
On December 4, 2018, Montero signed a minor-league deal with the Texas Rangers. Starting on June 17, Montero made appearances for the AZL Rangers, Frisco RoughRiders, and Nashville Sounds as he made his way back from TJS surgery. On July 22, 2019, the Rangers selected Montero's contract and promoted him to the major leagues. With Texas in 2019, Montero went 2–0 with a 2.48 ERA and 34 strikeouts over 29 innings.
