He attended Belmont Abbey College, where he hit .390 with a Division II leading 31 doubles in 2008 to earn All-Region honors. Was also an All-American and All-Region selection as a Freshman. He was drafted by the St. Louis Cardinals in the 10th round of the 2008 MLB Draft, becoming the first Belmont Abbey player to be drafted since 1972.

He began his career as an infielder in the rookie leagues and then moved to the outfield in 2010 with the Palm Beach Cardinals. He began 2011 with the AA Springfield Cardinals, where he hit .319 in 93 games with 19 home runs and 62 RBI and was selected as a Texas League mid-season All-Star.

Castellanos was traded to the Dodgers for Rafael Furcal on July 31, 2011 and hit .322 for the Dodgers AA affiliate, the Chattanooga Lookouts in 32 games. He was added to the 40-man roster after the season to protect him from the Rule V Draft and was promoted to the AAA Albuquerque Isotopes to start 2012, where the Dodgers started playing him more at second base.

On May 31, 2012 he was called up to the Majors for the first time, replacing an injured Matt Kemp on the active roster. He made his debut as a defensive replacement in the eighth inning and was later hit by a pitch in his first at-bat. Castellanos started the next game against Colorado and recorded his first hit as an RBI triple. He hit his first Major League home run in his final at-bat of the season, on October 3 against Jean Machi of the San Francisco Giants. In the 2012 season, he played in 94 games for the Isotopes, hitting .326 with 17 homers and 52 RBI, and 16 games with the Dodgers, hitting .174.

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