At the 1998 World Aquatics Championships in Perth, Western Australia, Mitchell finished fifth in the 200-metre breaststroke and then won silver at the 1998 Commonwealth Games in Kuala Lumpur, behind fellow Australian Simon Cowley, who won both breaststroke events. He gained selection to the 2000 Summer Olympics in Sydney after winning the 200-metre event and finishing third in the 100-metre event.