Sonko moved to France from Senegal at a young age and began his career with Olympique Noisy-le-Sec and AS Saint-Étienne, before turning professional with Grenoble.

Sonko helped Grenoble gain promotion from the Championnat National in 2000–01 and in 2002 he left to join English side Brentford.

He received a straight red card against Aston Villa for tripping Luke Moore in the penalty area, meaning Reading lost 2–1.

[13] However, on 20 January 2007, he injured knee ligaments when he fell awkwardly whilst playing for Reading in their 3–1 home win against Sheffield United,[14] and it was subsequently announced that he would miss the rest of the 2006–07 season.

[17] Sonko went on to play 16 matches for Reading in 2007–08 as they were unable to retain the Premier League status, being relegated in 18th place.

[18] Sonko ended the season on a sour note after he and teammate Emerse Faé were suspended by the club after refusing manager Steve Coppell's request to play in a reserve match.

[37] Sonko struggled with fitness and injuries during his time at Portman Road and was openly criticised by manager Paul Jewell after a 3–1 defeat against Leeds United.

[43] It was announced on 6 August 2015, that Sonko had returned to England and signed for Isthmian League Division One North club Harlow Town.

[44] In September of the same year, Sonko revealed he turned down a one-year contract with Scottish Premiership side Kilmarnock to remain with Harlow Town and stay close to his family.

He represented Senegal at U23 level, but declined his call-up to their African Cup of Nations squad of 2006, opting instead to help Reading's ultimately successful bid for promotion to the Premier League.

[48] Sonko was named in Senegal's provisional 38-man squad for the African Cup of Nations, which runs from 20 January to 10 February 2008.

He finally made his debut for Senegal on 12 January 2008 in their 3–1 win over Namibia in Dakar; he played the entire game in the centre of defence.