St. John's first major acting role was in 1993, when he starred as Jason Lee Scott, the Red Power Ranger, in Saban Entertainment's Mighty Morphin Power Rangers. Mighty Morphin Power Rangers was an instant success and quickly became one of the highest-rated children's television programs ever. However, Austin left the series midway through the second season, after a contract and pay dispute, and was replaced as the Red Ranger on the show by Steve Cardenas as Rocky DeSantos.
St. John later returned to the franchise as the Gold Ranger in Power Rangers: Zeo and in a supporting role as the recently retired-Ranger Jason Lee Scott in the second feature film Turbo: A Power Rangers Movie, along with his former castmate Amy Jo Johnson (Kimberly Hart, the original Pink Power Ranger). He also co-hosted the 1999 Power Rangers special titled "The Lost Episode," with his former castmate and close friend Walter Emanuel Jones (Zack Taylor, the original Black Power Ranger), in which they both discussed the history of Power Rangers up to that point and also presented the original, never-before-seen pilot of Mighty Morphin Power Rangers.
St. John's last Power Rangers appearance was in the tenth anniversary special episode of Power Rangers: Wild Force (2002) entitled Forever Red, where he joined nine other Red Ranger actors from the previous and the then-current season (minus Steve Cardenas as Rocky DeSantos) to battle and defeat the remnants of the Machine Empire from Power Rangers: Zeo. He once again suited-up as the original MMPR Red Ranger and was reunited with his former castmate Jason David Frank (Tommy Oliver, the franchise's longest-serving Power Ranger). St. John was also one of the guest speakers who appeared at the 2007 Power Morphicon convention held in Los Angeles, which celebrated the 15th anniversary of the Power Rangers franchise.
After several years of service overseas St. John began making convention appearances in 2014 including a return to Power Morphicon.
St. John will return to the acting world in an upcoming post-apocalyptic thriller called Survival's End.
In 2017, he will star in an upcoming short movie The Order alongside a host of other Power Rangers alumni, including Walter Emanuel Jones, Catherine Sutherland, David Yost, Johnny Yong Bosch, Paul Schrier, Karan Ashley, Steve Cardenas, Erin Cahill, Blake Foster, Nakia Burrise, Hilary Shepard Turner, Dan Southworth, Alyson Sullivan, Deborah Estelle Phillips, and Azim Rizk.
A indiegogo campaign from the upcoming movie entituled A Gift Of The Heart starring Austin St John and John Schneider (The Dukes Of Hazzard) was launched.