Carter attended Auckland Grammar School where he was a successful rower, competing twice in the Maadi Cup. He switched to triathlon when he realised he was not large enough to row at a senior level.
Carter won the bronze medal in triathlon at the 2002 Commonwealth Games and then went on to win the triathlon gold medal at the 2004 Summer Olympics, defeating his close friend, rival and fellow New Zealander, Bevan Docherty. Carter's time was 1:51:07.73, less than eight seconds faster than Docherty's. On 3 September 2006 in Lausanne, Carter won silver at the World Championships after finishing 17 seconds behind Tim Don. In October 2006, Hamish Carter won the Xterra World Championship in Maui, Hawaii defeating a field of more experienced off-road triathletes.
On 6 March 2007 he announced his retirement.