A defensive midfielder, Montgomery spent the majority of his career with Sheffield United, making 398 appearances across twelve seasons.

Montgomery had to wait until November 2001 before he scored his first goal for the club, coming in a 4–0 home victory over Birmingham City.

As a result of his hard work over the season, he was runner-up to Phil Jagielka in the club's Player of Year awards.

Losing his place in midfield to Michael Doyle he was confined to occasional substitute appearances as the season progressed.

[8] Montgomery still went on to make appearances as a Sheffield United player, including playing in the play-offs, in which the team lost to Huddersfield Town in the final at Wembley Stadium on penalties.

Montgomery ended his 12-year stay at Sheffield United by the cancellation of his contract through a mutual agreement on 31 August 2012.

This allowed him to join United's sister club Central Coast Mariners in the Australian A-League.

Montgomery was elected as vice-captain for the 2013–14 season, and made his debut as captain in a 1–0 win against rivals Sydney FC on 18 January 2014.