Born and raised in Sydney, Courtney started his career as a teenager with small roles in film and television, and studied acting at the Western Australian Academy of Performing Arts.

After early supporting roles in Hollywood projects, he gained recognition for playing Dauntless leader Eric Coulter in The Divergent Series (2014–2015), Kyle Reese in Terminator Genisys (2015), and supervillain Captain Boomerang in the DC Extended Universe films Suicide Squad (2016), The Suicide Squad (2021), and The Flash (2023).

He also portrayed the title character in the Melbourne Theatre Company's production of Macbeth (2017), and hosted the Australian reality competition series The Summit (2023).

[3] His father, Chris, worked for a state-owned electricity company, and his mother, Karen, was a teacher at Galston Public School, where Courtney and his older sister were students.

In 2008, he played a role in the Australian series Packed to the Rafters, then a guest lead in the popular All Saints, the comedy feature To Hell & Bourke and several short films.

[24] In 2019, Courtney played Wizz, a corrupt debt collector, in the independent crime comedy drama film Buffaloed, co-starring Zoey Deutch and Judy Greer.

Released to generally positive reviews from critics, Courtney's performance also garnered praise, with Sheila O'Malley of RogerEbert.com saying he plays the character "with gorgeous sinister sleaze".

[36] Courtney provided backing vocals on the Pinch Hitter song "All of a Sudden" from their debut album, When Friends Die in Accidents.