Born in Owerri, Imo, Nigeria, Kanu began his career at Nigerian league club Federation Works, before moving to Iwuanyanwu Nationale. After a notable performance at the U-17 World Championships he was signed by Dutch Eredivisie AFC Ajax in 1993 for an undisclosed fee. He made his Ajax debut that year and went on to score 25 goals in 54 appearances. Kanu also came on as a sub in Ajax’s 1995 Champions League final win over A.C. Milan.

In 1996, Ajax sold Kanu to Serie A side Inter Milan for around $4.7 million. That summer he captained the Nigerian team that won gold at the Olympics, and scored two late goals in the semi-finals against powerhouses Brazil to overturn a 2–3 scoreline into a 4–3 win in extra time. Kanu was also named African Footballer of the Year for that year.

However, soon after returning from the Olympics, Kanu underwent a medical examination at Inter, which revealed a serious heart defect; he underwent surgery in November 1996 to replace an aortic valve and did not return to his club until April 1997. In interviews, Kanu frequently cites his faith as a Christian, and has often mentioned this trying time of his career as an occasion when he prayed to God. Kanu’s experience also led to his founding the Kanu Heart Foundation, an organisation that helps predominantly young African children who suffer heart defects and whose work was expanded to provide aid for homeless children in 2008.

Kanu was born with a congenital heart defect, which impeded his aortic valve from closing properly; it was discovered and operated on in 1996. Although it was feared that he would not play again, and that it would affect his career, he made a full recovery. He undergoes an annual medical check-up for the condition. In March 2014, he successfully underwent corrective heart surgery once again, in the United States.

As well as winning the Olympic gold in the football event at 1996 Olympics in Atlanta, where he scored the winning goal in the 4–3 semi-final win over Brazil, his second goal of the match. Kanu participated in the 1998 and 2002 FIFA World Cups. On 24 June 2010, Kanu ended his international career following Nigeria’s exit from the 2010 World Cup in South Africa. Nigeria lost their group matches against Argentina and Greece, before a 2–2 draw with South Korea ended their stay in the tournament. He won 86 caps and scored 13 goals for his country and was the joint most capped Nigerian player of all-time alongside Muda Lawal, until Joseph Yobo surpassed both players in 2012, winning his 87th cap. Despite serving as a striker, he wore the number 4 shirt for Nigeria.

In February 1999, after just twelve league games and one goal for Inter, Kanu was signed by Arsenal for approximately £4.15 million. He made his debut for Arsenal, against Sheffield United in the FA Cup. With the score 1–1 and ten minutes to go, the Sheffield United goalkeeper Alan Kelly kicked the ball out of touch so that treatment could be given to Lee Morris. When the ball was thrown back into play by Ray Parlour, although it was intended for Kelly, Kanu chased the throw-in down the right wing unchallenged and centered the ball for Marc Overmars, who scored to make the match 2–1. After the match, Arsenal manager Arsène Wenger accepted that it was not correct sportsmanship and offered to re-play the match which was again won by Arsenal.

Despite the events overshadowing his debut, Kanu’s career was quickly revived at Arsenal. He scored his first goal for the club in the next round of the cup against Derby County, coming off the bench to net the only goal of the game. He quickly became known for his goal scoring prowess from the bench, scoring important goals against Sheffield Wednesday, Tottenham Hotspur and Aston Villa as a substitute. He became very popular among the fans for his two-fingered salute which started in 1999 against Middlesbrough – something that he later explained was based on the team’s nickname, The Gunners.

Kanu was named African Footballer of the Year for the second time in 1999. In the 1999–2000 season he scored 17 times in 50 matches for the Gunners, including a hat-trick in fifteen minutes against Chelsea to turn a 2–0 deficit into a 3–2 victory. In August 2001, Arsenal rejected a bid from Fulham of around £7m for Kanu. However, Kanu’s appearances for Arsenal gradually became less frequent, particularly after the emergence of Thierry Henry as Arsenal’s first choice striker with Kanu being mainly used as a substitute. Kanu then won the 2003 FA Cup with Arsenal. Along with other famous players, such as Henry and Robert Pires, Kanu is a member of the “Invincibles”, the Arsenal side that finished the 2003–04 season of the Premier League undefeated. He played 197 games for Arsenal, scoring 44 goals. In the summer of 2004, after his contract with Arsenal ended, he moved to West Bromwich Albion on a free transfer. In 2008, Kanu was voted 13th in the “Gunners’ Greatest 50 Players” poll.

Kanu’s own experience with a congenital heart defect moved him to set up the “Kanu Heart Foundation” in 2000, a foundation which aims to tackle homelessness, and which also built five hospitals in Africa to treat children with undiagnosed heart disease and provide them surgery.

West Brom had just been promoted to the Premier League for the second time in the space of two years. Kanu started as a regular for the club, making his debut in a 1–1 draw away at Blackburn Rovers on 14 August 2004. He scored his first goal for Albion on 18 September 2004, an 88th-minute equalizer in a 1–1 home draw against Fulham. In a match against Middlesbrough on 14 November 2004, Kanu was guilty of an incredible miss in injury time, with Albion 2–1 down. Kanu had sent a low cross over the bar from a yard away from the goal line. Manager Bryan Robson was seen in TV footage mouthing the words “How did he miss that?”, and Kanu’s howler was crowned “Miss of the Season” by many media outlets in their end-of-season reviews. Nevertheless, the 2004–05 season was ultimately a memorable one for West Brom, as they became the first club to avoid relegation from the Premier League after being bottom of the table at Christmas.

One of the most memorable games of the 2005–06 season for Kanu came with the visit of his former club Arsenal to The Hawthorns on 15 October 2005. Philippe Senderos put the visitors ahead in the 17th minute, but Kanu equalised shortly before half time. West Brom went on to win the match 2–1 with a spectacular strike from Darren Carter. It was their first home win over Arsenal since 1973, and the first time that they had come from behind to win a Premier League game. But such highlights were rare for Albion that season, and the club was relegated at the end of 2005–06. Kanu’s contract had expired, and he chose not to renew it. In his two years at The Hawthorns he made a total of 58 appearances – 16 of them as a substitute – and scored nine goals.

Kanu was a free agent following his departure from West Brom, and he signed for Portsmouth on a one-year deal shortly before the start of the 2006–07 season. Pompey had undergone a revival in the second half of the previous campaign, following the return of Harry Redknapp as manager, avoiding relegation by four points after being in serious danger at the turn of the year. At the start of the 2006–07 season, they were undefeated in their first five games, during which they did not concede a single goal. Kanu made his debut for Portsmouth as a substitute against Blackburn Rovers on 19 August 2006, the opening day of the 2006–07 Premier League season. He scored twice and missed a penalty. Kanu went on to finish the season as the top goalscorer for Portsmouth, with 12 goals altogether. He then signed a new one year deal with the club.

His first goal of the 2008–09 season put Portsmouth 2–0 up in their eventual 2–2 UEFA Cup draw with Italian club A.C. Milan. He later scored the winning goal against Bolton Wanderers, which ensured Pompey’s mathematical safety. It was his only Premier League goal of 2008–09. He re-signed with Pompey in August 2010, with an eye on becoming a coach when he retired. Kanu signed a three-year deal and kept the number 27 shirt, but was not a regular starter throughout the course of the season and only managed two goals.

During the 2011–12 season, his playing time was reduced, appearing only from the bench. After the departure of Steve Cotterill, and the arrival of Michael Appleton, he was also removed from the bench, due to fitness and injury problems. By May 2012, aided by a point deduction for entering administration Portsmouth had found themselves in a relegation battle which eventually ended in them relegated to Football League One. To cut costs the Administrator hoped to reach agreement to terminate the contracts of Kanu along with six other players due to them being the highest-earning players at the club. On 9 July 2012, it was announced that Kanu and Aaron Mokoena were expected to leave the club after failing to attend the first day back at pre-season training. Kanu threatened to sue Portsmouth for unpaid back wages. On 30 July, Kanu agreed to leave Portsmouth, but he reiterated that he was still in a dispute over the unpaid wages that the club owed him. In April 2013 he confirmed that he had dropped the case and let the club off the £3 million they owed him.