On December 11, 2009, Mejía signed a minor league contract with the Kansas City Royals organization that included an invitation to Spring Training.

Mejía split the year between the Naturals and the High-A Wilmington Blue Rocks, posting a .276/.337/.469 batting line with 16 home runs and 69 RBI.

In 2012, Mejía reached Triple-A for the first time with the Gwinnett Braves, and led the circuit in RBI (92), hits (150) and total bases (256), was second both in SLG (.517) and at-bats (489), and tied for second in home runs (24), ending third in extra-bases (57), fourth in OPS (.858), fifth in runs (72) and eight in batting average (.298), while tying for sixth in both doubles (32) and games played (126).

He also became the third consecutive Gwinnett Braves player to claim the award for the league's top rookie, joining Freddie Freeman (2010) and Julio Teherán (2011).

Mejía began the 2014 season with Gwinnett, but was released on April 26, 2014, after reaching an agreement to play with a team in Nippon Professional Baseball.

[9] Mejía led NPB's Pacific League with 34 home runs in 2014, finishing in a tie for the top spot with Takeya Nakamura.