Thirty-one-year-old Mills started his career with Derby County, turning professional at 18.
He went on to appear in sixty-four League and cup games for the Rams and during this time had loan spells at MK Dons and Walsall.
He joined Rotherham United during the summer of 2006, adding strength to the centre of defence.
However, he was loaned out to Crawley Town, then of the Conference National, in August 2007 to regain his fitness and his loan was extended until the end of October.
Mills returned to the Rotherham and was appointed club captain for the 2008/09 campaign.
He was released by Rotherham after the League Two play-off final defeat at Wembley in May 2010 and re-signed for Crawley.
He was named in the Conference team of the year for the 2010/11 season after Crawley won the title and were promoted.
Mills was released by Crawley in May 2012 and eventually stayed in the Conference by joining Macclesfield Town.
He re-joined Rotherham in August 2013 after a successful trial with the club and then signed for Bury after being released by the Millers and spent two months on loan with Cheltenham Town before leaving the Shakers at the end of last season.
Prior to joining the Saints, Mills spent time on trial with their league rivals Hednesford Town.
Also penning a deal with Brackley is former Chelsea and Leicester City defender and Colwyn Bay player-manager Frank Sinclair, who played a dozen games for the club after losing his job at Llanellian Road.