He trained at the Karen Clarke Group (now Summerscales Performing Arts), was taught by vocal coach Philosophy Wisdom and attended Songtime Arts. He moved to the United States in October 2006 where he studied at the American Musical and Dramatic Academy in New New York
He graduated in February 2008. He co-founded the Fundamental Theater Project in New York with Nicola Murphy in April 2010. While appearing as Marchbanks in George Bernard Shaw"s Candida at the Irish Repertory, New York, in April 2010, Underwood was asked to play the part of Alan Strang in a production of Equus at the John Drew, East Hampton, co-starring alongside Alec Baldwin.
In 2013, Underwood was cast in the eighth season of television series Dexter in the recurring role of Zach Hamilton, Dexter Morgan"s "protégé".
He subsequently joined the third season of Homeland as Leo Carras. Since February 2014 he has been portraying the twins Mark and Luke in the second season of The Following but after the death of Luke he only played Mark who had split identities in the third season of The Following.