Youth career Mannion joined
AFC Wimbledon at the age of 16. He was a key member of the
Under-18 team that knocked
Premier League academies
Watford and
Newcastle United out of the
FA Youth Cup before a fifth round defeat to
Chelsea.
Hull City In July 2016, Mannion signed for
Premier League club
Hull City on a three-year deal. Due to his age,
AFC Wimbledon were entitled to a compensation fee settled by a
tribunal. In May 2017, it was announced that
Hull City would pay an initial fee of £100,000, plus further payments of up to £100,000 for appearances for the first team, and up to £225,000 should he become an
Under-21 and full international in a competitive match. Mannion spent his first season with the club's Under-23 side, and was named on the bench in the
EFL Cup in August 2016. On 22 August 2017, he made his debut in a 2–0
EFL Cup defeat at
Doncaster Rovers. On 1 March 2019, Mannion had his contract with Hull City extended by one-year. Following the relegation of Hull City at the end of the
2019–20 season to the
EFL League One, Mannion was released by the club.
Plymouth Argyle (loan) Mannion joined
Plymouth Argyle on an emergency loan, on the same date of an
EFL Trophy game away to
Yeovil Town. This game quickly became his debut, the game ended in defeat for Argyle as they went crashing out of the cup 2–1 due to a late goal by
Sam Surridge. This turned out to be Mannion's only game for Argyle.
Aldershot Town (loan) On 9 August 2018, Mannion agreed to join
National League side
Aldershot Town on loan until January 2019. This was later extended to the end of the 2018–19 season.
Kidderminster Harriers (loan) He joined
National League North side
Kidderminster Harriers on loan on 29 October 2019 until 25 April 2020 and played in 17 games for them.
Pafos On 13 August 2020, Mannion joined
Cypriot First Division side
Pafos on a free transfer.
Cambridge United On 3 August 2021, Mannion signed a one-year deal with
Cambridge United.
Havant & Waterlooville (loan) On 22 October 2021, Mannion joined
National League South side
Havant & Waterlooville on a one-month loan deal.
Charlton Athletic On 10 June 2024, it was announced that Mannion would join
Charlton Athletic on a three-year contract with the option of a further year that would start when his deal at Cambridge United expired at the end of June 2024. == International career ==