Born in Swindon, England, Mills started his football career at Aston Villa's academy before opting to move to Southampton[4] in August 2006.
[3] At Southampton, alongside his brother Matthew, he progressed through the academy and reserve sides before being promoted to the first team ahead of the 2008–09 season.
[5] He made his Southampton first team debut as substitute in a League Cup match against Birmingham City on 26 August 2008.
[3] After returning from a loan spell at Scunthorpe United, Mills signed a new three-year contract in August 2009 after some good pre-season performances, featuring sporadically during the 2009–10 season.
[12] In the 2010–11 season, Mills lost his first team place under the management of Nigel Adkins and joined Doncaster Rovers on a one-month emergency loan deal on 19 October 2010,[13] making his debut the same day in a 3–2 defeat by Derby County.
[24] He made his first start for the Royals in a 2–1 defeat by Charlton Athletic, in the League Cup first round on 23 August 2011, coming off the bench for Ian Harte in the 62nd minute.
[26] On 6 November 2011, Mills was announced as Man of the Match after Reading defeated Birmingham City 1–0 thanks to a Noel Hunt winner.
[42] Mills made his Shrewsbury Town debut the next day, coming on as a substitute for Paul Parry in the 82nd minute, in a 1–1 draw against Brentford.
[43] On 29 March 2014, Mills received a straight red card after a foul on James Chambers, in a 1–0 loss against Walsall.
[44] After an unsuccessful appeal over his suspension,[45] Mills returned to the first team and made the total of thirteen appearances for the club.
[49] On 16 September 2014, Mills then provided assist for James Dayton to score the first goal of the game, in a 2–2 draw against Swindon Town.
[58] After suffering from ankle injury,[59] Mills made his first team return on 31 October 2015, in a 1–0 loss against Burton Albion.
[68] On 28 August 2021, in a 0–0 draw at Crawley, he was carried off the pitch with an ankle injury that was expected to prevent him playing for several months.
[72] He is the younger brother of retired footballer, Matt Mills, who was also his teammate when he played for Forest Green Rovers in League Two.