Niall Quinn played Gaelic football for the Perrystown, Dublin 12, club Robert Emmets. He also played underage football and hurling for Dublin. In July 1983, Quinn captained a Dublin Colleges GAA party on a one-month tour of Australia.

Aged 16, he played in the 1983 All-Ireland Minor Hurling Championship Final, and was offered a contract to play professional Australian rules football before settling on a career in soccer. He played Gaelic football for Co. Kildare club Eadestown after his retirement, winning a junior C county title in 2008.

He played as a youth for Irish club Manortown United, which was based at Greentrees Park, adjacent to Robert Emmets GAC (it not being unusual in Dublin to play two separate codes of football). After an unsuccessful trial at Fulham he signed professional forms with English club Arsenal in 1983. He was signed as a centre-forward, but also had a brief spell as a centre-half for the Arsenal third team. After scoring 18 goals in 18 reserve matches in the first half of the 1985–86 season, Quinn was included in the first-team squad for a match against Liverpool. Quinn scored in the match as Arsenal recorded a 2–0 win. He made a further 11 league appearances for Arsenal that season, but failed to score as they finished seventh in the league. The end of the season brought Quinn his first call-up to the Republic of Ireland national team.

Quinn played in the qualifiers for the 1986 UEFA European Under-18 Football Championship, and made his Irish international debut at under-17 level against Northern Ireland at Seaview in a 6–1 friendly win in January 1985, the first ever fixture between the two nations. Quinn scored a hat trick as did Eamonn Dolan. However Arsenal refused permission for Quinn to travel to the 1985 FIFA World Youth Championships.

Quinn made his senior debut as a substitute against the host nation in the Iceland Triangular Tournament in 1986. Quinn played for his country at two World Cups, in 1990 and 2002; he missed the 1994 FIFA World Cup because of injury. Quinn was also a member of the Irish squad that participated in the 1988 European Football Championship playing just once, as a substitute in the Republic of Ireland’s 1–0 win over England in Stuttgart.

Quinn found himself playing under a new manager for the following season, as George Graham was appointed in place of Don Howe. Quinn had a regular place in the side that season, appearing in 35 league games and scoring eight goals. He also collected a Football League Cup winner’s medal as Arsenal triumphed 2–1 over Liverpool. However, after Arsenal signed another target man, Alan Smith, in the 1987 close season, Quinn struggled to get into the team. Over the next three seasons he managed a total of just 20 league appearances and five goals – his three appearances in 1988–89 not being enough for a title medal. Quinn’s lack of opportunities led him to submit a written transfer request at the start of the 1989–90 season. In total he scored 20 goals in 94 appearances for Arsenal, of which 81 were starts.

Manchester City manager Howard Kendall signed Quinn for £800,000 in March 1990, shortly before the transfer deadline. He marked his debut with a goal, in a 1–1 draw against Chelsea at Maine Road. He scored 22 times in his first full season, and he went on to spend six years at the club, scoring 78 goals in 245 appearances; his time at City was hampered by a cruciate ligament injury in 1993–94. Although he returned to the side the following season, he managed just eight goals from 35 games.

Quinn scored the equaliser against the Netherlands in the 1990 FIFA World Cup which allowed the Republic to progress to the second round of that tournament. In the qualifiers for the 2002 FIFA World Cup, he scored against Cyprus on his 35th birthday to break the all-time goalscoring record, then held by Frank Stapleton. In the tournament proper, his header set up Robbie Keane’s late equaliser against Germany, which was the only goal Germany conceded before the final. In the second-round, with the Republic behind 1–0 to Spain, it was a foul on Quinn that led to Ireland’s last-minute penalty, converted by Robbie Keane, which tied the game and brought extra-time, but the Republic lost 3–2 in the resulting penalty shootout.

Quinn has his own song titled “Niall Quinn’s Disco Pants”. The song was originally created by Manchester City supporters during a night out on a pre-season tour in Penola, Italy, in 1992. There had been a bust-up with City team-mate Steve McMahon and Quinn had removed his torn and bloodied shirt and was dancing with Rick Holden wearing just a pair of cut-off jeans. He was “hardly aware” that there were a group of hardcore City fans watching and they treated him to “the first performance of the song that will follow me till the end of my career.”

His most notable game for City was 20 April 1991 when he scored early on and saved a penalty as City beat Derby County 2–1, relegating Derby in the process. City goalkeeper Tony Coton had been sent off before half time for fouling Dean Saunders to concede the penalty. At this time teams rarely named goalkeepers as substitutes, so Quinn replaced Coton in goal. Other notable games included the Manchester derby on 7 November 1993, in which he scored twice in the first half to put City 2–0 up against United by half time, although a remarkable United comeback saw City lose 3–2.

In the 1993 close season, Everton made a bid to sign Quinn and a further bid was made early in the 1993–94 season, but both bids were rejected and Quinn remained at Maine Road for a further three seasons. A cruciate ligament injury sustained in a match against Sheffield Wednesday in November 1993 caused Quinn to miss the majority of the 1993–94 season, and prevented him from playing in the 1994 FIFA World Cup. He returned at the start of the 1994–95 season, but the partnership forged by Uwe Rösler and Paul Walsh in his absence meant he was not always a starter. In attempt to reduce the wage bill, Manchester City tried to sell Quinn in the 1995 close-season, but a proposed move to Lisbon club Sporting fell through after failure to agree contractual terms.

Quinn finished his career with a highly successful spell at Sunderland, joining the north-east club in August 1996 for a club record £1.3 million, although he missed six months of his first season due to a knee injury – similar to the one which ruined his World Cup chances three years earlier. Before his injury, he had got off to a fine start to his Sunderland career, finding the net twice on his debut in a 4–1 win at Nottingham Forest. In his absence from September to March, Sunderland struggled and although he was back in action by the end of the season, they were relegated.

His partnership with striker Kevin Phillips, signed in the 1997 close season, was one of the most prolific in the Football League in the late 1990s/early 2000s (a combined 194 goals for both players in all competitions from 1997-98 to 2002-03) and helped the club to regain promotion to the Premiership for the 1999–2000 season. In March 1999 Quinn again had to play in goal, this time replacing the injured Thomas Sørensen in a game against Bradford City. In similar circumstances to when playing for Manchester City against Derby County in 1991 Quinn scored and then went in goal, and kept a clean sheet to help his side win. He also has the distinction of being the first player to score at Sunderland’s Stadium of Light, against Manchester City in 1997. He became a local legend at Sunderland, winning both the Sunderland and North East Sportswriters’ Player of the Year awards in 1999 after scoring 21 goals in Sunderland’s record-breaking Division One title-winning season. His final appearance for Sunderland came on 19 October 2002 against West Ham.

The song was adopted by Sunderland fans and released as a single by the club’s dedicated fanzine A Love Supreme. It reached no. 56 in the UK Singles Chart in April 1999.

Quinn had a testimonial match between Sunderland and the Republic of Ireland in 2002. He donated the entire proceeds to charity, an act for which he received a number of awards, including an honorary MBE. Instead of receiving an appearance fee for the game, all the players received a letter from a sick child. Quinn played for both teams during the match, which raised over £1 million. The Republic of Ireland won the match 3–0.

Quinn retired in 2003 at the age of 37, taking a brief coaching role at Sunderland. Quinn has also made appearances as a television pundit and commentator for televised matches involving his former teams with Sky Sports. He made a single league appearance for Thai Premier League side BEC Tero Sasana in March 2006 to promote the league and the side’s link with Arsenal.

After the tournament, he announced his retirement from international football, amassing 92 caps. At the time, he was his country’s all-time top scorer with 21 goals; this record was later surpassed by Robbie Keane in October 2004.

Heading the Drumaville Consortium of wealthy Irish businessmen, in June 2006, Quinn successfully brokered a deal to buy a controlling stake in Sunderland. In July 2006 Quinn became the chairman and manager of Sunderland. The deal was finalised on 27 July 2006, with sufficient shares being sold to the consortium in order for them to take complete control.

His managerial career did not get off to a good start as the team lost its first four league games in a row. On 22 August, Sunderland played Bury away in the League Cup where they lost 2–0. After the match Quinn said that a new manager would be appointed by Sunderland’s next game. Quinn stepped to one side (to continue in his role as Sunderland chairman) paving the way for Roy Keane to take charge. This was highly unexpected considering the huge rift between the two arising from Keane’s infamous ejection from the 2002 World Cup. Keane was appointed manager of the club on 28 August 2006. The appointment matured into a great success, with Sunderland clinching an immediate Premier League comeback as Football League Championship champions. Quinn made substantial amounts of money available for buying new players, as he had a declared ambition to establish Sunderland as a top club.

Quinn has received praise for his relationship with Sunderland fans. This was highlighted in March 2007 when, during his tenure as chairman, Quinn paid over £8000 to allow around 100 Sunderland fans to travel home by taxi when stranded at Bristol airport after being ejected from their scheduled flight home. The incident happened on Saturday night when Quinn, Sunderland staff and a group of supporters boarded an easyJet flight from Bristol to Newcastle, following Sunderland’s 1–0 win at Championship promotion rivals Cardiff. After spotting Quinn, many of the fans broke into song, serenading Quinn with ‘Niall Quinn’s Disco Pants’. EasyJet staff, angered by the boisterous behaviour, ejected the fans from the plane and Quinn, outraged at the fans’ treatment, also left the plane. Quinn, seeing there were no other flights available, decided to commandeer a fleet of taxis and minivans in order to return the fans home.

In 2008, he received the James Joyce Award of the Literary & Historical Society in University College Dublin.

In 2010, Quinn was named a patron of the Sir Bobby Robson Foundation.

In October 2011 Ellis Short replaced Quinn as chairman of Sunderland. Quinn was appointed as Director of International Development on behalf of the club and remained in this role until stepping down in February 2012 and after six years involved with the running of the club saying “Everything is in place for Sunderland to really make a statement, which was always my aim”.

In 2012, Quinn started commentating for Sky Sports and normally commentates alongside Martin Tyler. He won, also in 2012, the North East Football Writers’ Association’s Personality of the Year for 2011. In November 2013, Quinn was bestowed with the Freedom of Sunderland award by the city of Sunderland.

In January 2020, Quinn was appointed as interim deputy chief executive officer of the Football Association of Ireland, choosing to go without salary until the financial future of the organisation could be secured. In September 2020 he stepped down from his interim CEO role.