Martínez began in the Washington Nationals organization, commencing his professional career in 2006. That year, he played for the Vermont Lake Monsters, Savannah Sand Gnats and Potomac Nationals, hitting a combined .265 with 11 stolen bases in 82 games. In 2007, he played for the Hagerstown Suns and hit .250 with 13 stolen bases in 116 games. He played for the Potomac Nationals and Harrisburg Senators in 2009, posting a .259 batting average with 10 steals. In 2010, he played for the Senators and Syracuse Chiefs, hitting a combined .272 with 11 home runs and 23 stolen bases.

He was selected by the Phillies from the Nationals in 2011 as a Rule 5 Draft pick. He made the Phillies' Opening Day roster as a reserve player. He made his major league debut on April 3 against the Houston Astros and recorded his first major league hit and RBI. He hit his first major league career home run on July 17 against the New York Mets. He spent the season acting as a utility infielder and outfielder, recording a .196 batting average with 3 home runs and 24 RBI in 89 games. Due to his remaining on the club's roster for the entire regular season, he remained in the organization thereafter per Rule 5 regulations.

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