He starred as Jason the Red Ranger in Mighty Morphin Power Rangers for 79 episodes from 1993-94, ending with the episode "Opposites Attract" (though Jason continued to be featured via body doubles, voice doubles, and stock footage before being written off in "The Power Transfer"). He reprised his role of Jason in Power Rangers: Zeofor 17 episodes in 1996 when Jason temporarily took over the Gold Ranger powers. After his departure from Zeo, St. John soon after returned as Jason yet again in the 1997 film Turbo: A Power Rangers Movie, along with former cast mate Amy Jo Johnson.
After the film, St. John appeared as the co-host of the 1999 Power Rangers special "The Lost Episode," along with former castmate Walter Jones. St. John did not appear in the Power Rangers again until 2002, when he reprised his role of Jason for the last time in Power Rangers: Wild Force,in the episode "Forever Red." He co-starred with nine other former Red Ranger actors.