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.