Flint began his career in the Central Midlands Football League with Pinxton before stepping up to the Conference Northin the summer of 2008 when he joined Alfreton Town. Flint spent three months of his debut season at Alfreton on loan with Northern Premier League Premier Division side Matlock Town. The adverse weather prevented Flint from playing many games but the central defender scored in the 5–0 victory over Leigh Genesis in January 2009. The following month saw Flint play in the 1–1 draw against Ilkeston Town, and scored an own goal in the 2–0 loss at Whitby Town. On 26 February 2009, Matlock manager Mark Atkins confirmed that Flint was among three players to have departed the club along with goalkeeper Andy Richmond and forward Simon Barraclough. Flint spent the rest of his Alfreton career within its first team set-up helping reach the Play-Offs during the 2009–10 season. He impressed many visiting scouts with bids being turned down from Derby County, Luton Town and ten other clubs according to Alfreton chairman, Wayne Bradley.
In January 2011, Colchester United manager John Ward confirmed that his club had been in talks with Flint over a move to the Essex club. Ward also announced that Flint was scheduled to travel to Wiltshire the next day to discuss a possible move with Swindon Town with Danny Wilson. Days later Swindon Town announced the signing of Flint on a -year contract and was issued the number 16 shirt which had been recently made available after the departure of on-loan Manchester City forward David Ball. Flint made his Swindon debut in the 1–1 draw with Rochdale and featured in the following fixture against Leyton Orient which resulted in a 3–0 loss for the Wiltshire club. In March 2011, Flint returned to Alfreton Town on a loan until the end of the 2010–11 season. New Swindon manager Paul Hart was impressed with Flint's attitude in training but had decided to select more experienced players such as Andy Frampton instead. Upon rejoining Alfreton, Flint was immediately handed the captaincy and led the club to promotion from the Conference North. On 6 August 2011, Flint scored his first goal against Crewe Alexandra. He was a consistent performer in Town's 2011–12 title winning season. Flint scored twice in a 3–0 win against Stevenage at the County Ground on 20 April. The victory secured Swindon's place in the 2012–13 League One play-off.
Flint signed for Bristol City on 11 June 2013 for a fee of £300,000. Due to a sell on clause his previous club Alfreton Town received 20% of the fee. He made his debut on 3 August in a 2–2 draw against Bradford City at Ashton Gate. He scored his first goal for the club on 21 September in a 3–2 defeat to former club Swindon Town. On 8 February, he scored his second goal for the club in a 1–1 draw against Oldham Athletic. He scored again in the following game three days later in a 3–1 win against Leyton Orient. Flint briefly sparked a hostile relationship with fans of his former team Swindon by stating in an interview that he did not wish for them to achieve promotion. On 3 May 2015, in the final game of the 2014–15 season, he scored his first ever hat-trick in an 8–2 victory over Walsall, bringing his total for the season to 15. Aden Flint was awarded Player of the Year for the 2015–16 season, in which he helped Bristol City narrowly avoid relegation back to League One. Flint featured in every game as Bristol City reached the semi finals of the 2017-18 EFL Cup with wins over Premier League opponents Watford FC, Stoke City, Crystal Palace F.C. and Manchester United. Flint scored in the semi-final defeat against Premier League leaders Manchester City.
On 27 June 2018, Flint joined Championship club Middlesbrough for an undisclosed fee believed to be in the region of £7m, signing on a four-year contract.
Cardiff City
Flint joined Cardiff City on 19 July 2019 on a three-year contract He made his debut on the opening day of the 2019–20 season in a 3–2 defeat to Wigan Athletic. In his second appearance, he scored his first goal for the club, scoring the opening goal in a 2–1 victory over Luton Town.
International career
Flint's form at Alfreton Town during the early 2010–11 campaign earned him his first international call-up when Paul Fairclough named his within his 16-man England C squad for the friendly with Wales national semi-professional football team. He played the first 45 minutes in the 2–2 draw at Latham Park in Newtown, Wales. Flint was capped for a second and final time in October 2010 when he played the entire 90 minutes of England C's 1–0 away win at the Kadriorg Stadium, Tallinn against an Estonia U-23's in the International Challenge Trophy. When Flint signed for Swindon Town in January 2011, he became ineligible for selection for the England C squad so was unable to make the final squad for the International Challenge Trophy.
