Montero signed with the St. Louis Cardinals as an international free agent in August 2014.[1] He made his professional debut in 2015 with the Dominican Summer League Cardinals where he hit .252 with three home runs and 30 RBIs in 57 games. He played 2016 with the Dominican Summer League Cardinals, batting .260 with one home run and 26 RBIs in 61 games, and 2017 with the Gulf Coast Cardinals where he compiled a .277 batting average with five home runs and 36 RBIs in 52 games.
Montero started 2018 with the Peoria Chiefs where he was named a Midwest League All-Star.[2] After slashing .321/.381/.529 with 15 home runs, 69 RBIs, and 28 doubles in 102 games, he was promoted to the Palm Beach Cardinals in August.[3][4] In 24 games for Palm Beach, he hit .286 with one home run and 13 RBIs.[5] Following the season, Montero was named the Midwest League Most Valuable Player.[6]
Montero began 2019 with the Springfield Cardinals, and was placed on the injured list in April.[7] He returned to play in May, but was once again placed on the IL at the end of the month. He was activated once again in July. Over 59 games with Springfield, Montero hit .188 with seven home runs and 18 RBIs. He was selected to play in the Arizona Fall League for the Glendale Desert Dogs following the season.[8] Montero was added to St. Louis' 40–man roster following the 2019 season.[9] Montero did not play a minor league game in 2020 since the season was cancelled due to the COVID-19 pandemic.[10]
He returned to the Isotopes to begin the 2022 season.[15] On April 29, 2022, Montero was recalled and promoted to the major leagues for the first time after Kris Bryant was placed on the injured list.[16] On August 6, Montero hit his first career home run, a solo shot off of Arizona Diamondbacks starter Merrill Kelly.[17] In 53 games during his rookie campaign, he slashed .233/.270/.432 with six home runs and 20 RBI. Montero played in 85 games for the Rockies during the 2023 season, hitting .243/.290/.426 with career–highs in home runs (11) and RBI (39).[18]
Montero played in 61 games for Colorado in 2024, batting .206/.268/.305 with four home runs, 28 RBI and a -1.6 Wins Above Replacement. On June 30, 2024, Montero was designated for assignment by the Rockies.[19] He cleared waivers and was sent outright to Triple–A Albuquerque on July 4.[20] Montero elected free agency following the season on November 4.[21]