[1] He began his racing career in 2011 in the North American Nissan GT Academy, a televised competition that took the fastest sim racers in Gran Turismo to compete in real cars to win a professional racing contract with Nissan.

After finishing second in the competition, Johnston competed in the IMSA GT3 Cup Challenge the following year, claiming 14 podiums from 15 races and winning the championship.

[3] In 2018, Johnston switched to rallying, competing alongside co-driver Alex Kihurani.

[4] Johnston's first rallying win came at the 2020 Rallye Automobile Monte Carlo, where he won the RC4 class,[5] becoming the first American with a class win at the event.

[6] Johnston and Kihurani drive the Citroën C3 Rally2 for Saintéloc Junior Team.