Mervue United Manning started his senior career at
Mervue United, playing for the club's under-19 side before progressing to the club's first team. He made his debut as a 16 year-old against
Athlone Town in the second tier of
Irish football, the
League of Ireland First Division. When Mervue United withdrew from the national league, after the O'Connor report recommended that one club represent the city of Galway, Manning signed for Galway F.C. (later renamed
Galway United).
Galway United in 2014 After his debut season at
Mervue he moved to fellow
First Division side
Galway United for the
2014 season. Manning and Galway had a brilliant season as they went on to win promotion to the
Premier Division via the
Promotion/relegation playoffs, with Manning scoring in both legs of the final as United brushed aside Premier Division outfit
University College Dublin in a 5–1 aggregate win.
Queens Park Rangers In January 2015, Manning was signed by
Queens Park Rangers on a two-and-a-half-year contract. The then manager
Harry Redknapp stated he was delighted that Manning had signed for Rangers after having received interest from other Premier League clubs. After Redknapp departed QPR, Manning initially did not feature in the first team under managers
Chris Ramsey or
Jimmy Floyd Hasselbaink. But, with the arrival of
Ian Holloway, he was given his first team debut on 31 December 2016 against
Wolves a game that Rangers went on to win after having had a run of six consecutive losses. He performed so well that he was hailed by his manager as having been "outstanding". Manning described his own debut as a dream come true, especially as he only received his squad number the day before. He impressed in his next two games, which were all victories, and he was thus awarded a new contract tying him to the club until 2019. He rounded off the month by scoring his first league goal against
Fulham on 21 January, in a game which ended in a 1–1 draw against their west London rivals.
Rotherham United (loan) On 16 August 2018, Manning joined
Rotherham United on a loan deal until the end of the 2018–19 season. Manning was recalled by QPR in January 2019. He made 18 appearances and scored four goals for Rotherham during his time on loan at the New York Stadium.
Swansea City On 16 October 2020, Manning joined fellow Championship side
Swansea City for an undisclosed fee, signing a three-year contract. He made his debut for the club on 27 October, starting in a 2–0 victory over
Stoke City, a victory that saw Swansea move into the automatic promotion places. In February 2023, Swansea manager
Russell Martin confirmed that Manning would depart the club at the end of the
season with the defender having no intentions of signing a new contract.
Southampton On 11 July 2023, Manning joined
Southampton on a free transfer, signing a four-year contract. Manning made his professional debut for the club on 4 August 2023 in a 2–1 away victory against
Sheffield Wednesday. Following his fifth
yellow card of the campaign during a 2–1 away victory against
Hull City, he was suspended for one game. He made his first
Premier League appearance on 5 October 2024 in a 3–1 defeat against
Arsenal. Southampton were ultimately relegated from the Premier League, but Manning outlined his desire to stay at the club. He made 27 appearances in all competitions throughout the season. Manning scored another free-kick on 30 August in a 2–2 draw with
Watford. ==International career==