His professional debut was Brothers of the Head, a feature film about conjoined twin brothers in a punk rock band. Treadaway took time out from his course at LAMDA to work on Brothers of the Head, and graduated in 2006. Harry took on other professional commitments while still at drama school including Miss Marple: Sleeping Murder for ITV television, and a reading of a new play, Myrna Molloy for Operating Theatre Company in 2006.
Since then, he has continued to work on TV, movies and stage. He was in the TV movie Recovery(2007), playing the son of characters played by David Tennant and Sarah Parish, and played the role of Mark Brogan on the series Cape Wrath (2007, known as Meadowlands in America). He has acted in the horror film The Disappeared (2008), directed by Johnny Kevorkian, and also in the science fiction-fantasy film City of Ember (2008). And he made his stage debut in Over There, a new play by Mark Ravenhill, alongside his twin brother Luke Treadaway at the Royal Court Theatre in 2009. In 2013 Harry joined actor Johnny Depp in the remake of The Lone Ranger.
Coming in 2020, you can see Harry in the newest addition to the Star Trek franchise, Star Trek: Picard.