His credits include Control (2007), City of Ember (2008), Fish Tank (2009), Pelican Blood (2010), Flight of the Storks (2012), Mr. Mercedes (2017–2018), The Crown (2019), Star Trek: Picard (2020), Deceit (2021), and The Chemistry of Death (2023).

[4] Inspired by a love of Eddie Vedder[4] and with support from their secondary school drama teacher,[5] Harry and his twin brother, Luke Treadaway, formed a band called Lizardsun.

[7] The same year, he made his stage debut in Over There, a play by Mark Ravenhill, alongside his twin brother Luke at the Royal Court Theatre in 2009.

[2] His part was that of Brady Hartsfield, a brilliant young psychopath who decides to engage in a cat-and-mouse game with Gleeson's Bill Hodges.

[10][11] In 2021, he played Detective Inspector Keith Pedder in the Metropolitan Police drama Deceit, about a real-life undercover operation investigating the killing of Rachel Nickell in 1992.