While playing for Bastia, he was selected as a part of France under-16 national team. He later chose to represent Cameroon, his country of birth. He played as a centre-back but later made several appearances as a defensive midfielder. He was highly regarded in this position by numerous coaches and supporters of Bastia. During his time at Bastia, he attracted attention from numerous clubs, including Internazionale, Juventus, Manchester United, Lyon and Middlesbrough. Unwilling to sell their prized asset, Bastia agreed to loan him to the Gunners for one season only in 2005. Despite being linked heavily with Lyon, he was sold to Arsenal for a £1 million transfer fee. During his time at Bastia, Song made a total of 32 league appearances.

Song impressed Arsenal manager Arsène Wenger as a trialist during Arsenal’s pre-season training camp in Austria, and Arsenal secured his services on loan for the 2005–06 season on 11 August 2005. Arsenal agreed to a £1 million fee in June 2006 and signed him to a four-year contract. Song made his Premier League debut during Arsenal’s 2–0 win against Everton on 19 September 2005 as a substitute. He played in several UEFA Champions League matches and made several Premier League starts near the end of the season, when first team players were injured or being rested.

Song was first capped by Cameroon in 2005. He was named in the Cameroon squad for the 2008 Africa Cup of Nations. He played for Cameroon in their first group match, a 4–2 defeat against holders Egypt; he replaced Stéphane Mbia at half time, joining his uncle Rigobert on the pitch. For the duration of the tournament he was a revelation in defence for Cameroon, earning a Man of the Match award in Cameroon’s semi-final, but then going off injured in the final against Egypt. He was nonetheless named in the Tournament Best XI, alongside compatriot Geremi.

Song scored his first goal for Arsenal against Liverpool in Arsenal’s 6–3 win at Anfield in the League Cup quarterfinals on 9 January 2007.

On 30 January 2007, it was confirmed that Charlton Athletic had signed Song on loan until the end of the 2006–07 season. Although he impressed, Charlton were relegated and Song returned to Arsenal.

On 21 October 2008, Song scored his first goal in European competition for Arsenal against Fenerbahçe in the UEFA Champions League. Song’s first league goal, and third in all competitions for the Gunners, was scored against Wigan Athletic on 11 April 2009 in which he took on several players and then coolly finished in the bottom corner. Arsenal won the match 4–1.

Song began the season well, playing in all but one of Arsenal’s first 12 league games. He was voted third in the Arsenal.com player of the month award for October, behind winner Cesc Fàbregas and runner-up Robin van Persie. After putting in consistently commanding performances at the heart of midfield, allowing Arsenal’s creative attackers to operate effectively, Song established himself as one of the first names on the team sheet. On 25 November 2009, Song signed a new long-term contract with Arsenal, lasting until 2014. On 30 December 2009, he scored his second goal in the league for Arsenal when he completed the scoring in a 4–1 win over Portsmouth at Fratton Park.

Song continued to be one of the first names on the team sheet, as he made a superb start to the 2010–11 season, establishing himself in front of the back four. He scored the 1,000th Arsenal Premier League goal under Arsène Wenger in a 4–1 win against Bolton Wanderers on 11 September 2010. On the downside, the following game he was sent off for two bookable offences. The strong midfielder has become a more adventurous element in the Arsenal midfield, helping the forward line with his movement. On 28 August 2010, Song completed the full 90 minutes in the game against Blackburn Rovers in a 2–1 win at Ewood Park. On 19 October 2010, he scored the first goal of the match for Arsenal against the Ukrainian side Shakhtar Donetsk in a 5–1 win for Arsenal. On 24 October 2010, Song scored his third goal of the season, the second goal at Eastlands against Manchester City in a 3–0 Arsenal victory. On 30 October 2010, he scored a vital 88th minute diving header for Arsenal from a cross by Gaël Clichy at the Emirates Stadium against West Ham United in the league, leading to an important 1–0 victory for Arsenal. In February 2012, Song had his car impounded by Hertfordshire Police after he was caught speeding and failed to provide relevant documentation. On 27 December 2010, Song scored the first goal in a 3–1 victory over Chelsea at the Emirates. During the 2010–11 season, he made 42 appearances in total, scored five goals and contributed three assists in all competitions for the Gunners.

In 2011, Song became an ambassador for Grassroot Soccer, an international non-profit organisation working through football to stop the spread of HIV.

With the departure of Cesc Fàbregas to Barcelona, Song was assigned to provide the assists for the club alongside Aaron Ramsey. During a league match at Newcastle United, Song stamped on Joey Barton, resulting in him being charged with improper conduct and given a three-match ban. On 24 September 2011, Song scored his first goal of the season in a 3–0 home victory against Bolton. He provided an assist to Robin van Persie following a dribble for the first goal in a Champions League match against Borussia Dortmund on 23 November to secure a place for Arsenal in the last 16. Wenger said in November:

Song again assisted Van Persie – who scored a volley – during a match against Everton which also proved to be the only goal as the 125th anniversary game ended 1–0 to Arsenal. Song also showed his assisting skills in the following matches: in an FA Cup match against Leeds United, Song played a through ball past four Leeds players to the returning legend, Thierry Henry. The game ended 1–0, sending Arsenal into the fourth round of the FA Cup. He then provided two assists in back to back matches against Tottenham at home for Theo Walcott and also the winning goal for Van Persie against Liverpool at Anfield with a delightful long-ball in the dying minutes of the match. The matches ended in 5–2 and 2–1 victories respectively, highlighting the importance of Song’s presence in the team not only as an anchorman in the middle but as a playmaking midfielder. Song played an overhead pass to Theo Walcott and claimed an assist against Aston Villa on 24 March 2012. Arsenal went on to win the game 3–0. His partnership with striker Van Persie was enhanced in a match on 5 May 2012 as he performed his seemingly trademark chip to assist the Dutchman in netting the equaliser against Norwich City. The game ended 3–3 and it was his 12th assist of the season. He was voted in second place as the Arsenal player of the season by the fans.

On 20 August 2012, Song signed a five-year contract with Barcelona for a £15 million transfer fee. He made his Barcelona debut against Real Madrid in the 2012 Supercopa de España on 29 August 2012 as a substitute for the last 15 minutes, finishing the match with a perfect passing statistic. He played 20 matches in Barcelona’s La Liga-winning campaign in his first season, and scored one goal, in a 3–1 home win against Real Zaragoza on 17 November 2012.

Song was an unused substitute in the 2014 Copa del Rey Final, which Barcelona lost 2–1 to Real Madrid at the Mestalla Stadium.

In August 2014, Song was subject to transfer speculation linking him with a season long loan deal to Liverpool.

On 30 August 2014, Song joined West Ham United on a season-long loan. He was unveiled to the fans on the same day before their home game against Southampton. Song made his debut against Hull City as a second-half substitute on 15 September 2014 in a 2–2 draw. He played 31 games for West Ham including three in the FA Cup. Praised for his early season form, his performances deteriorated towards the end of the season as West Ham’s form also slumped. In August 2015, West Ham co-chairman David Sullivan, announced an agreement with Barcelona to sign Song on a three-year permanent contract subject to a medical. His season-long loan to West Ham was confirmed on 1 September with West Ham having the option to extend the loan for a further season.

Song participated in his second FIFA World Cup at the 2014 tournament in Brazil. In the team’s second group match, Song was sent off for violent conduct after striking Mario Mandžukić as Cameroon were knocked out, losing 4–0 to Croatia. Song later apologized to Mandžukić, and to the people of Cameroon for being sent off. FIFA punished Song with a three-match ban from competitive internationals, as well as fining him US$22,300. The match is being investigated for possible match-fixing after allegations in a German newspaper. In December 2014, despite being in-form for his club West Ham United, he was omitted by Cameroon coach Volker Finke from the squad for the 2015 Africa Cup of Nations.

On 6 January 2015, Song announced his international retirement, aged 27 and earning 49 caps, citing his wish to focus on rebuilding his domestic career with West Ham.

On 21 July 2016, Rubin Kazan reached an agreement to sign Song from Barcelona, subject to a medical. On 1 August 2016, Song officially joined the Russian side on a free transfer after his contract with Barcelona had been terminated.

In January 2018, Song trained with his old club Arsenal after being allowed to find a new club by Rubin, but was still registered with Rubin when the Russian Premier League continued on 2 March 2018 after their winter break. He was removed from their league registration list on 13 March 2018. It was reported on 2 April 2018 that Rubin sent a complaint to the FIFA Dispute Resolution Chamber, demanding 40 million euros from Song for breach of contract.

Song signed with Swiss Super League club Sion on a free transfer on 14 August 2018. In March 2020, Song was one of nine players sacked by Sion for refusing to take a pay cut during the coronavirus pandemic.

In November 2020, Song joined Djiboutian club Arta/Solar7 on a permanent deal.