Signed by the Yankees in 2006, Montero became one of the best prospects in baseball. After he debuted in MLB, the Yankees traded him to the Mariners for Michael Pineda. After his rookie year in 2012, Montero struggled with the Mariners in 2013 before ending his season with a 50-game suspension for his involvement in the Biogenesis scandal. He spent most of the 2014 and 2015 seasons in the minor leagues. The Blue Jays claimed Montero before the 2016 season. Montero agreed to terms on a minor league contract with the Baltimore Orioles on January 3, 2017.

On July 2, 2006, Montero, an amateur free agent, signed a professional contract with the New York Yankees of Major League Baseball (MLB), receiving a $1.6 million signing bonus.Baseball America considered Montero to be the best player available and the best power hitter in the 2006 international free agent class. In 2007, at age 17, Montero made his professional debut in Minor League Baseball for the Gulf Coast Yankees of the Rookie-level Gulf Coast League. As a result of his performance, the Yankees invited Montero to spring training in 2008, where he hit a home run in his only at bat before being assigned to minor league camp. He spent the 2008 season with the Charleston RiverDogs of the Class A South Atlantic League, where he batted .326 with 17 home runs and 87 runs batted in (RBIs), with two stolen bases.Baseball America rated Montero as the Yankees' second best prospect after the 2008 season.

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