Gary O'Neil became Norwich's third signing of the 2014 pre-season when he signed a two year deal on 5th August 2014.

O'Neil began his career at Portsmouth, progressing from their youth team and making his debut in January 2000 against Barnsley as a sixteen year old. After loan spells at Walsall (2003) and Cardiff (2004) he went on to make 174 appearances, scoring 16 goals before joining Middlesbrough for approximately £5million on 31st August 2007. He made 109 appearances for Boro, scoring eight goals, before returning south to join West Ham in January 2011. He played 48 games for the Hammers scoring three goals

on 7th August 2013, he joined QPR on a one year deal. He went on to make 29 appearances as he helped them to promotion from the Championship.

O'Neil made his Norwich debut as a substitute in the 3-0 win over Watford on 16th August 2014.

He made 24 appearances in 2014/15 as City won promotion back to the Premier League, including coming on as substitute in the 2-0 Championship Play-off final victory over Middlesbrough on 25th May 2015

He made 29 appearances for City in 2015/16 as the Canaries were relegated from the Premier League.

Despite offer of a new contract with Norwich, O'Neil opted to join Bristol City signing a two year contract witj Bristol City on 9th June 2016. He was released at the end of the 2017/18 season having made 36 appearances for the Robins, scoring once.

On 3rd August 2018, O'Neil signed for Bolton on a short term deal until January 2019. This was extended on 18th January until the end of the season. On 19th April 2019, Bolton were relegated from the Championship following a 2-0 defeat to Aston Villa. It was their 14th defeat in 18 games.

O'Neil left Bolton on 1st July 2019. He trialled with Aberdeen in Autumn 2019 but didn't play during the 2019/20 season due to an Achilles injury which led to his retirement.

On 10th August 2020 O'Neill was appointed as Liverpool's Assistant Manager for their Under-23 squad. On 25th February 2021 he was appointed as first team coach at Bournemouth along with Joe Jordan, working under new manager Jonathan Woodgate. He remained in the role under new manage Scott Parker and they led Bournemouth to promotion to the Premier League on 3rd May 2022 with a 1-0 win over Notts Forest that secured them second place in the Championship.

On 30th August 2022 he was appointed caretaker manager of Bournemouth on August 30. This followed Bournemouth’s sacking of former Canary Scott Parker after the Cherries lost 9-0 to Liverpool. O’Neil’s first two games saw a 0-0 draw with Wolves followed by a 3-2 away win at Notts Forest. On 27th November 2022, having picked up 13 points from 11 league games as caretaker manager, O'Neil was appointed as Bournemouth's permanent head coach, signing an initial 18-month contract.

Despite leading Bournemouth to a 15th place finish in the Premier League, winning 10 of his 34 games in charge, he was sacked on 19th June 2023. Bournemouth owner Bill Foley said "This has been a difficult decision, It has been made with great consideration to best position ourselves ahead of the coming season. Gary will go on to have a long career as a head coach or manager, but we feel that, at this moment in time, a change is in the best interests of this football club." He was replaced by Andoni Iraola who had managed Rayo Vallecano.

On 9th August 2023 he was appointed as manager of Wolves replacing Julen Lopetegui on a three-year deal."Gary's a highly motivated young coach with strong principles," said Wolves' sporting director Matt Hobbs."It's an opportunity to do something different with an up-and-coming British coach and from the people we met, Gary was clearly the right person."

O'Neill was sacked by Wolves on 15th December 2024 after their 2-1 defeat at home to Ipswich on 14th December. The defeat was their 11th in the Premier League this season - they have had only two wins - and leaves the Black Country club 19th, four points from safety. They have also struggled defensively, having conceded a league-high 40 goals and let in two or more goals in 13 of their 16 matches. Wolves chairman Jeff Shi said: "We're very grateful to Gary for all of his effort, dedication and hard work during his time at the club, and we wish him and his team the best of luck for the future."

In May 2025 he was spoken to about potentially succeeding Johannes Hoff Thorup in the Carrow Road hot seat, along with a host of other candidates

Gary has been capped at Engand Under-19, Under-20 and Under-21 levels. He captained the England Under-20 side at the World Youth Championships in Dubai in 2003.