Growing up in Adelaide and being coached by Glenn Beringen at the South Australian Sports Institute, Mitchell made his debut at the 1994 Commonwealth Games in Victoria, British Columbia, Canada. He was the 1995 silver medalist in the 200-metre breaststroke at the world short course championships in Rio de Janeiro. He won selection to the national team for the 1996 Summer Olympics in Atlanta, after dead-heating with Phil Rogers to win the 200-metre breaststroke at the Australian Championships. In December of same year, Mirchell broke the world record for Men's 4×100-metre medley relay and the Australian &amp; Commonwealth Shortcourse record for 200-metre breaststroke. He swam slower at the Olympics, finishing in eleventh place.