He has since starred in the Christmas horror film Better Watch Out (2017) and the romantic comedy The Broken Hearts Gallery (2020) and portrayed Steve Binder in Baz Lurhmann's Elvis (2022).

Montgomery completed his acting degree at the Western Australian Academy of Performing Arts at Edith Cowan University in 2015.

In 2015, Montgomery appeared in the music video for "Old Souls" by Australian deathcore band Make Them Suffer directed by Jason Eshraghian.

[10] In 2017, Montgomery joined the cast for the second season of the Netflix series Stranger Things as Billy Hargrove, an abusive and unpredictable teenager.

On November 6, 2017, Montgomery was announced to be joining The True History of the Kelly Gang based on the novel of the same name by Peter Carey, alongside Russell Crowe and Nicholas Hoult.

[12] On October 19, 2022, he was announced to appear in a leading role in Went Up the Hill, co-scripted and directed by Samuel Van Grinsven.

[17] Montgomery has opened up about having OCD in an Instagram post about an episode of his podcast, stating how it is a condition that both "fuels" and "hinders" him.