Sam Underwood was born in Woking, Surrey and attended the Winston Churchill School. He trained at the Karen Clarke Theatre Group (now Summerscales Performing Arts). He was taught by vocal coach Phil Wisdom and attended Songtime Theatre Arts.

He moved to the United States in 2006 where he studied at the American Musical and Dramatic Academy in New York.

After graduating he went on to appear as Marchbanks in George Bernard Shaw's Candida at the Irish Repertory Theatre, New York. In April 2010, he was asked to play the part of Alan Strang in a production of Equus at the John Drew Theater at Guild Hall of East Hampton, co-starring alongside Alec Baldwin.

In 2013, Sam 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.

Beginning in February 2014, Sam took on the dual roles of twins Mark and Luke Gray in the second season of the Fox murder mystery The Following. But after the death of Luke, he was left playing Mark – who had split identities – in its third and final season.

In 2017, Sam went on to join the cast of the hit AMC thriller series Fear the Walking Dead in its third season as Jake Otto, the enigmatic son of the colony leader.

In 2019, he was cast as Adam Carrington in The CW television series Dynasty, a reboot of the 1980s series of the same name.

When this podcast was recorded, he was playing the title role of Holly Martins in the brand new musical production of The Third Man directed by Trevor Nunn at the Menier Chocolate Factory in London.