Douglas Henshall was born in Glasgow, Scotland, in November 1965. His mother was a nurse and his father was a travelling salesman. He is the youngest of three children, with two older sisters. He grew up in Barrhead and attended Barrhead High School. While still at school, a friend suggested he join the Scottish Youth Theatre in Glasgow. After leaving school, he moved to London to study at the Mountview Theatre School. After training, he returned to Glasgow, where he joined the theatre company 7:84. Eventually he moved back to London, where he earned critical acclaim for his stage work, particularly for "Life of Stuff" at the Donmar Warehouse (1993) and "American Buffalo" at the Young Vic (1997). One of his first successful film roles was as Edgar in "Angels and Insects" (1995), after which he appeared in "Orphans" (1998), "The Man with Rain in His Shoes" (1998), "Hearts and Bones" (2001) and "It's All About Love" (2003). He also appeared in many television series and is known for his roles in "Psychos" (1999), "The Child in the Corner" (1999), "Loving You" (2003) and "Frances Tuesday" (2004).

Douglas also took part in BBC radio productions, including the role of Romeo in "Romeo and Juliet" (1999), David in "The Long Goodbye" (2002), Jack Parlabane in "Bampot Central" (2004) and Richard in "Richard III" (2004). In the summer of 2002, Douglas returned to the London stage, where he played Mikhail Bakunin in Tom Stoppard's new trilogy of plays "The Coast of Utopia" at the National Theatre. Throughout 2004 he continued his stage work, playing John Proctor in Arthur Miller's "The Crucible" at the Crucible Theatre in Sheffield and Thomas Huxley in Crispin Whittell's "Darwin in Malibu" at the Hampstead Theatre. In 2005, Douglas made his West End debut at the Lyric Theatre on Shaftesbury Avenue, playing Biff Loman in the critically acclaimed production of Arthur Miller's "Death of a Salesman" alongside Brian Dennehy.