He spent the entire 2004 season playing with Subiaco and was a part of their premiership team. Impressive WAFL form continued into 2005 and he was selected by Fremantle to make his AFL debut at the MCG against Melbourne in Round 6. His poise and composure in defence saw Mundy maintain his position in the side for all 17 remaining games for the season. In round 14 he was recognised by the AFL by being nominated for the AFL Rising Star award. In the final voting he came 3rd behind Brett Deledio and Ryan Griffen with 15 votes. He also was awarded the 2005 Beacon Award as the best young talent on the Fremantle list.
Mundy started the 2008 season in good form, and was shifted to a midfield role late in the season in order to utilise his kicking ability. In 2009 he played his 100th game and was only the eighth player in league history to play them consecutively from debut. He played 124 consecutive games from his debut until he was a late withdrawal due to stomach cramps before the Round 17, 2010 match against the Western Bulldogs.
2010 was his best season to date, and he was recognised by being named in the 40-man squad for the 2010 All-Australian team and also won the Doig Medal as Fremantle's best player for the year. At the end of the 2010 season, there was much speculation that Mundy would not renew his contract with Fremantle and would instead either return to play in Melbourne, or be recruited by the new Gold Coast Football Club. However, he rejected all other offers and re-signed with Fremantle in September 2010 for an additional four years.