Born at the Royal Devon and Exeter Hospital in Exeter, Devon, Treadaway was brought up in Sandford, Devon. His father is an architect and his mother is a primary school teacher; he has two brothers, older brother Sam (an artist) and slightly older twin Luke. Treadaway and his twin brother Luke attended Queen Elizabeth's Community College in Crediton, Devon, where they played in the twice Devon Cup winning Rugby Union team.
Inspired by a love of Eddie Vedder and with support from their secondary school drama teacher Phil Gasson, the twins formed a band called Lizardsun with Matt Conyngham and Seth Campbell. They also both joined the National Youth Theatre.
His professional debut was Brothers of the Head, a feature film about conjoined twin brothers in a punk rock band. Harry played Tom Howe, the band's rhythm guitarist and songwriter, and his brother Luke played Barry Howe, the lead singer. During rehearsals and throughout the shoot, Harry and Luke were connected to each other for fifteen hours a day, wearing sewn-together wetsuits or a harness. They also slept in one bed to simulate the conjoined nature of their characters. The Treadaways performed all tracks featured in the film themselves live on stage, as well as recording nine tracks for the sound-track album.
Treadaway took time out from his course at LAMDA to work on Brothers of the Head, and graduated in 2006.
Treadaway 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 graduating, he has taken on work such as Recovery for Tiger Aspect (playing the son of characters played by David Tennant and Sarah Parish) and as Mark Brogan on the Channel 4 series Cape Wrath (known as Meadowlands in America). In Control Harry plays Joy Division drummer Stephen Morris. In 2008 he appears in the Channel 4 drama The Shooting of Thomas Hurndall and a short film by Sam Taylor-Wood.
He has acted in horror film The Disappeared, directed by Johnny Kevorkian, and science fiction-fantasy film City of Ember.
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.
Treadaway is also credited as a songwriter, after writing the piece Sink or Swim which he and Luke performed both on film and on the soundtrack of Brothers of the Head. Also he performed his song "Raise This Up" in "Brothers of the Head" as a solo performance during the scene in which Tom Howe's girlfriend breaks his heart.
In 2011, he appeared in The Last Furlong, filmed in Ireland. He starts as the title character James Furlong.
He played Victor Frankenstein in the Showtime TV series Penny Dreadful, starting in May 2014.
He has recently filmed a role through motion capture alongside an ensemble cast of other actors for the upcoming video game Squadron 42, the single player campaign of Star Citizen, a spiritual successor to Wing Commander.