From Melbourne, Simpson originally played for Eltham in the suburban Diamond Valley Football League, and he also played under-18 football for the Northern Knights before being recruited by North Melbourne at the 1993 National Draft. He made his senior debut for the club during the 1995 season, and the following season played in North Melbourne's grand final win over Sydney, having also been nominated for the Rising Star award earlier in the season. A midfielder, Simpson also played in the club's 1999 premiership side. In 2002, he won the club best and fairest award (the Syd Barker Medal), and was also named in the All-Australian team. Later in his career, Simpson served as captain of North Melbourne, for five season from 2004 to 2008, and he retired at the end of the 2009 season after 306 games. From 2010 to 2013, prior to being named senior coach of West Coast, he served as an assistant coach at Hawthorn.
He received the club captaincy in the 2004 season and led the Kangaroos into the finals the following year.
In one of his best games of his career, in Round 5 2007 against Geelong at Skilled Stadium, Simpson racked up a game high 41 possessions in the Roos 16 point win. He also kicked a goal.
Played his 250th game against Carlton at Carrara a Kangaroos home game at the Gold Coast. North Melbourne won the game 22-13-147 to 20-10-130. Simpson had 20 disposals from the game. When he played his 200th game his fellow players forgot to chair him off the field, they did not make the same mistake for his 250th.
Simpson has only missed 7 games. He is one of only seven players to reach the 250 mark for the club.
On 5 November, Adam Simpson announced he was stepping down as captain but would continue on playing in season 2009. The new captain is Brent Harvey.
The skipper and star midfielder only narrowly missed recording a fifth consecutive top-three finish in the Best and Fairest of 2005 finishing 4th-a single vore behind 3rd and 2 votes shy of second. Missed two games with a puncutred lung in 2005, but still racked up more than 400 disposals in his typically industrious way.
Simpson announced his retirement on 27 July 2009 effective after 31 July Friday night match, due to North Melbourne's inability to make the finals. Now retired from the professional ranks, he has taken up the opportunity to help out his first club, Eltham, where he will play a number of games within their senior program in 2010.
In 2010 Simpson took up an assistant coaching role with the Hawthorn Football Club and as of the 2014 AFL season, he will be the senior coach of the West Coast Eagles after John Worsfold stood down from the role after the Eagles had a disappointing 2013 season. He beat red-hot favourites in Scott Burns and Peter Sumich at the coaching position