From Melbourne, Simpson played junior football for Eltham and the Northern Knights before being recruited to North Melbourne at the 1993 National Draft. He made his debut during the 1995 season, and won a premiership the following year, during which he was also nominated for the AFL Rising Star award. Another premiership followed in 1999, and in 2002, Simpson was named in the All-Australian team and also won North Melbourne's best and fairest award, the Syd Barker Medal. He was appointed club captain in 2004, and held the position until stepping down at the end of the 2008 season, with his span including a preliminary final in 2007. Simpson played his 300th game in 2009, the third North Melbourne player to do so, and finished his career towards the end of the season. He was appointed coach of West Coast in October 2013, replacing John Worsfold.