Gardner came through Aston Villa’s youth system and turned professional on 31 January 2005. Gardner made his Premier League debut on 26 December 2005, when he came on as a substitute for Steven Davis against Everton at Villa Park. He scored his first professional goal for Villa in the 3–1 away win on 14 April against Middlesbrough. He also netted a contender for goal of the month on the last day of the season away to Bolton Wanderers. After a series of appearances Villa Manager Martin O’Neill entered contract talks with Gardner.

In August 2007, Gardner signed a new four-year deal, which ran until June 2011. On 1 October 2007 he scored twice in two games with two free kicks. First he scored away to Tottenham Hotspur in a 4–4 draw and then the winner against West Ham United five days later with a deflected attempt.

On 1 December 2007, Gardner scored the opener in a home match against Arsenal. Villa went on to lose the match 2–1. Gardner has often played in the right-back slot, and impressed both his manager, Martin O’Neill, and captain, Gareth Barry, who said that Gardner could be a key player for years to come.

Gardner made just one appearance for Villa in the 2009–10 season because of a groin injury. On 4 January 2010 it was announced that Birmingham City were preparing a £3.5 million bid. However it was revealed that Aston Villa had turned down the bid for the player. However, the club later offered a second deal including add-ons that was accepted by Aston Villa. He made exactly 80 appearances for Villa in all competitions.

Gardner signed a four-and-a-half-year contract with Birmingham City on 26 January 2010 for a fee reported as £3m plus £500,000 in add-ons. He made his debut on 7 February as a 68th-minute substitute, and set up the first of Kevin Phillips’ goals as Birmingham came back to beat Midlands rivals Wolverhampton Wanderers 2–1. He scored his first goal for the club against Everton on 13 March. Towards the end of the season Gardner was preferred to Sebastian Larsson on the right of midfield. Restored to his favoured central position, he scored twice in the first home game of the 2010–11 season as Birmingham came from behind to beat Blackburn Rovers 2–1, and scored again in the next game away at Bolton Wanderers. Gardner’s winning goal in extra time against West Ham United took Birmingham City through to the 2011 Football League Cup Final, and he was in the starting eleven as Birmingham won their first major trophy since 1963. He was the team’s top scorer in the 2010–11 season with eight League goals, ten in all competitions, as Birmingham were relegated to the Championship.

Gardner was born in Solihull and raised in the Yardley district of Birmingham, where he attended Cockshut Hill School. After transferring to Birmingham City in 2010, he said he was a lifelong Birmingham City supporter. His younger brother Gary Gardner also played for England at youth level and turned professional with Aston Villa. In June 2012, Gardner donated £500 to the family of a disabled toddler after a chance meeting in a department store; this was only revealed after the Stoke-on-Trent family told a local newspaper.

Gardner signed a three-year deal with Sunderland for an undisclosed fee, believed by BBC Sport to be about £6 million, on 30 June 2011. Gardner made his debut as a substitute in the home defeat to Newcastle United on 20 August, and scored his first competitive goal on 18 September 2011 with a deflected shot that looped over goalkeeper Asmir Begović as Sunderland beat Stoke City 4–0 at home to secure their first win of the season. Gardner fell out of favour at Sunderland, and was linked with a departure in the January transfer window. However, he was re-united with former boss O’Neill when he replaced Steve Bruce as Sunderland manager in December 2011, and this prompted a change in fortune for Gardner and the Black Cats. Gardner was deployed at right back against Manchester City on New Year’s Day 2012 due to an injury crisis, and helped Sunderland to a shock 1–0 win against the league leaders. Two days later Gardner, again playing at right back, scored his second goal for Sunderland, direct from a free kick 30 yards away from goal in their 4–1 away win at Wigan. Gardner was rumoured to be homesick at Sunderland in the press, and was linked with a move back to the Midlands. Sunderland rejected a loan bid for Gardner from former club Birmingham City on 19 January. Gardner scored a stunning volley from 25 yards late on in Sunderland’s 2–0 win over Swansea City on 21 January.

On 20 May 2014, Gardner signed a three-year deal with West Bromwich Albion, to begin when his Sunderland contract expired at the end of the 2013–14 season. He scored his first goal for the club in a 1–0 victory against his former club Aston Villa.

On 11 January 2017, Gardner returned to Birmingham City on loan until the end of the season, after which the transfer would become a three-year permanent deal with an option for a fourth year. The fee was undisclosed, but the Birmingham Mail believed it to be in excess of £1 million. The deal was made permanent on 11 May 2017. He made a further 53 appearances in all competitions, and was appointed player-coach in November 2019.

At the end of the 2019–20 season, Gardner decided to retire from playing in order to take up a permanent position as Birmingham’s first-team coach. On 9 July 2020, he was appointed caretaker manager alongside Steve Spooner following the departure of Pep Clotet, remaining in this joint position until the end of the extended Championship season, which had been extended due to the coronavirus pandemic.