After returning from Manchester, Legg was offered a trial with his home town club Swansea City, playing in a reserve fixture against Cardiff City and, in August 1988, he was offered a professional contract by manager Terry Yorath, starting his professional career at the relatively late age of 22. A stress fracture of his leg meant he was forced to wait for his debut, eventually making his first appearance in a 2–0 defeat to Bristol City.

He went on to make over 150 appearances at Vetch Field and helped win the Welsh Cup in 1989 and 1991 as well as featuring in the team’s European Cup Winners’ Cup campaigns, playing against Panathinaikos and AS Monaco. During his time with Swansea, Legg’s long throw-in ability gained significant attention and he was credited with a Guinness World Record in 1992 after taking part in a long throw competition arranged by the BBC television show Record Breakers held at Wembley Stadium in 1992 against Dave Challinor and Neil Thompson. He recorded a throw distance of 44.6m, a record that stood until the 1996–97 season when it was beaten by Challinor who recorded a throw of 46.3m.

He was sold to Notts County in July 1993 for a fee of £275,000, where he was part of the side that won the Anglo-Italian Cup during the 1994–95 season.

Along with Paul Devlin, Legg was sold to Birmingham City in February 1996. Originally signed by Barry Fry, he spent two years, winning his first cap for Wales on 24 April 1996 in a 2–0 defeat to Switzerland but was forced off after 30 minutes due to injury. In the final year of his contract during the 1997–98 season, Legg began negotiations over an extension but, after failing to agree terms, was dropped to the reserve side by manager Trevor Francis. He was instead allowed to join Ipswich Town on loan as cover for the injured Bobby Petta and after a month Ipswich manager George Burley attempted to negotiate a permanent deal, which later collapsed.

Soon after, Jason Bowen, a friend of Legg’s joined Reading and, thanks to Bowen’s praise of the club, he agreed a deal to sign for the Berkshire based side in February 1998. Just several weeks later manager Terry Bullivant was sacked and replaced by Tommy Burns who soon fell out with several players including Legg and Bowen. Burns ignored the group of players, who later became known as the “gang of five”, and refused to let them be a part of the first team, resulting in them taking part in separate training sessions taken charge of by reserve team manager Alan Pardew, who took over from Burns following his dismissal. Pardew later commented that “”I wish they had stayed. Andy would certainly be in my team now and Jason showed he had a lot of talent.”

After a brief spell on loan at Peterborough United, he joined Cardiff City in 1998. Legg was initially unpopular with fans due to his previous spell with South Wales rivals Swansea City, even being sent death threats, former teammate Winston Faerber had his finger sliced open after opening one of Legg’s letters which contained a razor blade. However, he eventually became a fan favourite and was awarded the club’s player of the season award in the 1999–2000 and 2000–01 seasons. In 1999, Legg’s wife Lucy noticed a lump on his neck which was later diagnosed as a non-malignant tumour. With the 1999–2000 season coming to an end, Legg decided to play on for the remainder of the season, the only people aware of his condition at the club being manager Billy Ayre, physio Mike Davenport and club doctor Len Noakes, before having the tumour removed at the Princess of Wales Hospital in Bridgend in May 2000.

He left the club in July 2003 after being asked to take a 70% pay cut for the 2003–04 season, stating “It’s a big wrench to leave Cardiff, I’m absolutely gutted […] I think I deserve better than a 70% pay cut”. As a result, his final game for Cardiff was in the 2003 Football League Second Division play-off Final, helping the club win promotion to Division One.

Following his departure, Legg joined Peterborough United as a player-coach, making his debut on the opening day of the season in a 3–4 defeat to Hartlepool United. As assistant manager to Steve Bleasdale at Peterborough United he featured briefly in the TV series Big Ron Manager alongside Ron Atkinson and Barry Fry. In 2004, a second lump appeared in his throat and, after seeing a specialist, he decided to again wait until the end of the season before undergoing surgery to remove it. However, during a match against Oldham Athletic on 22 January 2005 he was hit in the neck by an elbow from Neil Kilkenny. Reacting angrily, Legg was later sent off for a retaliatory foul on Kilkenny.

Feeling pain from his neck for the first time, Legg booked himself in for the operation in April 2005, making his last appearance for Peterborough on 9 April 2005 in a 1–0 win over Blackpool, a match which would later turn out to be his last match as a professional. He underwent surgery several days later at Queen’s Medical Centre in Nottingham. The operation to remove it was successful, although he was forced to undergo a 28 session course of radiation therapy, and Legg was able to return to football, despite having believed that he would be forced to retire.

He made his return with Maesteg Park on 1 February 2006 during a 2–0 win over Bettws. After short spells playing for Newport County and Llanelli, Legg joined Conference North side Hucknall Town. In January 2007, he was appointed manager of Hucknall Town. He remained manager until September 2007, when he resigned following six consecutive defeats at the start of the 2007–2008 season. He returned to play for Llanelli, and was appointed manager of the side in April 2009, replacing Peter Nicholas. At the age of 42, Legg was part of the Llanelli side that inflicted a shock defeat on Scottish Premier League side Motherwell in the first qualifying round of the Europa League in July 2009.

Already a holder of a UEFA A coaching licence, Legg began studying for his UEFA Pro Licence in June 2010.

Legg returned to football as manager of Bangor City in August 2016 but was sacked from the job three months later. In September 2016 he was appointed under-23 manager at his former club Cardiff City. As part of an academy shake-up, Legg left his position on 25 November 2019.