He attended Belmont Abbey College, where he hit .390 with a Division II leading 31 doubles in 2008 to earn All-Region honors.

He began 2011 with the Double–A 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.

On May 31, 2012, Castellanos was promoted to the major leagues for the first time, replacing an injured Matt Kemp on the active roster.

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.

The team abandoned the plan to turn him into an infielder, as they had attempted the previous year, and he played in the outfield the whole season.

On November 11, 2014, the New York Mets signed Castellanos to a minor league contract that included an invitation to spring training.

[citation needed] On July 28, 2015, Castellanos joined Yomiuri Giants of Nippon Professional Baseball (NPB) as an outfielder and was given the number 93.