O'Malley was born in Cleveland, Ohio, and was brought up by his single mother, who is of Irish ancestry. He grew up Catholic. He graduated from Saint Ignatius High School in Cleveland, Ohio and Carnegie Mellon University. O'Malley became friends with actors Josh Gad and Leslie Odom Jr. at Carnegie Mellon University and remains good friends with them.
O'Malley made a small cameo appearance in On the Run in 2004. His best-known screen appearance was in the 2007 film adaptation of Dreamgirls. O'Malley also performed the song "Cadillac Car" on the soundtrack, Dreamgirls: Music from the Motion Picture.
In 2018, he became a series regular on Lifetime's American Princess.

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