[1][2] A midfielder, Johnston spent the 2011 season playing for the Casey Scorpions, Melbourne's affiliate team in the Victorian Football League (VFL).

However, he managed only six games for Casey's senior team, instead spending most of the season in the VFL reserves (a third-tier competition).

[4] After failing at his AFL ambitions, Johnston signed up with ProKick Australia in 2012, an academy headed by Nathan Chapman for Australian players looking to pursue a career in the United States.

[5] As a freshman, Johnston led all Big Ten Conference punters with a 44.0 per punt average and ranked 16th-best nationally.

[13] He made his NFL debut in the 2018 season opener against the Atlanta Falcons and had six punts for 313 net yards in the 18–12 victory.

The Texans signed undrafted rookie Ty Zentner to handle punting duties while Johnston recovered.