In one of her earliest international senior performances, she placed fourth at the 1999 All-Africa Games. She was the junior champion at the 1999 African Junior Athletics Championships, but failed to finish in the event at the 2000 World Junior Championships in Athletics. She was selected for the heptathlon at the 2001 World Championships in Athletics and placed 13th overall. Senior success came the following year as she took the bronze medal at the 2002 Commonwealth Games and then won the heptathlon gold medal at the 2002 African Championships in Athletics. She failed to finish at the 2003 World Championships in Athletics. In 2004, Simpson became African Champion for a second time and followed this result with a ninth place at the 2004 Athens Olympics.