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.[1] In 2013, he secured a full scholarship with Ohio State.[5] As a freshman, Johnston led all Big Ten Conferencepunters with a 44.0 per punt average and ranked 16th-best nationally. He was named a second-team freshman All-American by College Football News later that year. In his final three years at OSU, he was part of the 2014 national championship team, earned second and first-team honors in his junior and senior years respectively, and was named Eddleman-Fields Big Ten Punter of the Year. He also ranked academically in the Big Ten his final two years.[6]
On 1 September 2018, Johnston made the Eagles 53-man roster for the 2018 season.[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.[14]
On 24 March 2020, Johnston was re-signed to a one-year, $660,000 contract by the Eagles.
On 25 August 2023, Johnston pulled a calf muscle. The Texans signed undrafted rookie Ty Zentner to handle punting duties while Johnston recovered.[16] On 31 August, Johnston was placed on injured reserve.[17] He was activated on 7 October.[18]
Pittsburgh Steelers
On 19 March 2024 Johnston signed a three-year, $9 million contract with the Pittsburgh Steelers.[19]