Cowell was born in Sydney and grew up in the beachside suburb of Cronulla.
He credits his mother and high school drama teacher with encouraging him to explore his creative side.
He attended Charles Sturt University in Bathurst to complete a Bachelor of Arts in Theatre/Media.
Cowell won the Patrick White Playwrights' Award for his third play, Bed along with a collection of other awards.
2007
Cowell's acting work in film include roles in the 2007 crime drama Noise, the World War 1 war film Beneath Hill 60, the romantic comedy I Love You Too.
and a notable role in Avatar: The Way of Water as Captain Mick Scoresby.
2008
He won some acclaim for his portrayal of the title role in Bell Shakespeare's 2008 Production of Hamlet and acted in Sydney Theatre Company's production of True West, directed by Philip Seymour Hoffman, in 2010.
2009
His play Ruben Guthrie showed at the Belvoir St Theatre in 2009 to sell-out houses.
2010
In 2010 Cowell published his first novel, How it Feels.
Cowell dated Rose Byrne for six years until they parted ways amicably in early 2010.
2011
It had a new production at La Boite Theatre in 2011, starring Gyton Grantley and directed by David Berthold.
2013
His play Happy New was performed in London in 2013, starring Joel Samuels, Lisa Dillon and William Troughton.
It had previously premiered in Australia.
2015
The Sublime (Melbourne Theatre Company) was shortlisted for the Nick Enright Prize for Playwriting in the New South Wales Premier's Literary Awards 2015.
2017
In 2017 he starred as Galileo Galilei in the Young Vic's production of Bertholt Brecht's Life of Galileo.
Cowell played the enigmatic Tom on Australian cable TV's Love My Way, for which he also wrote several episodes, and played Todd for the first two seasons on Life Support on SBS TV, for which he also wrote sketches.
In 2017 Cowell joined the cast of the HBO series Game of Thrones in Season 7 as Harrag, an Ironborn sea captain allied to Theon Greyjoy.
In 2024, it was announced that Cowell had begun filming for ABC drama Plum.
