Johnstone was born in Invercargill, New Zealand, where he attended Verdon College. In the early 2000s, Johnstone and Jaquie Brown worked together at C4. A few years later, they created a production company, Young, Gifted & Brown, to make the New Zealand sitcom The Jaquie Brown Diaries, based on a satirical fictionalised version of Brown. It began airing in 2008 with Johnstone serving as writer and director.
Johnstone directed episodes of the first season of The New Legends of Monkey, airing in 2018. That same year, he signed on to direct the comedy horror film, M3GAN. Producer James Wan chose Johnstone for the job after admiring his ability to balance horror and comedy elements in Housebound. M3GAN was released on 6 January 2023 to commercial and critical success. It grossed over $30 million on its opening weekend and garnered positive reviews.

Filmography:

Film

Year	Title	Director	Writer
2014	Housebound	Yes	Yes
2022	M3GAN	Yes	No