He appeared in small roles in Australian dramas Rake, Sea Patrol and Underbelly: The Golden Mile, before he was cast as Fish Lamb in Cloudstreet.

Johnstone-Burt went on to star in Carelesss Love and he took on the role of Constable Hugh Collins in ABC1's Miss Fisher's Murder Mysteries.

[4] Johnstone-Burt told TV Week's Erin Miller that he chose a career in acting because he was not good at anything in high school, saying "I was a bit of a clown and liked to muck around and make people laugh, and then I went to my first drama class and thought, 'Well, amazing, I can do this for the rest of my life.

'"[4] His parents supported his career choice and after leaving Barker College, Johnstone-Burt auditioned for the National Institute of Dramatic Art (NIDA) aged 18.

[8] A week before he graduated from NIDA, Johnstone-Burt was approached to play the brain damaged Fish Lamb in the television adaptation of Tim Winton's Cloudstreet.

[10] He told a The Daily Telegraph reporter, "I supposed I walked out thinking I did a good job – seeing how I worked with [director Matthew Saville] and taking his direction.

"[10] To prepare for the role, Johnstone-Burt visited a home for people with intellectual disabilities and he then spent four months shooting the miniseries in Western Australia.

[17] The actor called his character, Jamie Sharpe, "a super-creepy stalker guy" and said he had worked a lot with Ada Nicodemou (Leah Patterson-Baker).

[19][20] Johnstone-Burt appeared in the Network Ten drama The Wrong Girl as Vincent, the brother of lead character Lily played by Jessica Marais.

[29] The couple announced their engagement in November 2019,[30] and they married in the Blue Mountains in March 2021, having postponed the ceremony twice due to the COVID-19 pandemic.