He played for Martigues, Chamois Niortais, Le Mans Union Club 72, RC Lens, Rangers, Hull City, Larissa and Sapins, as well as the Gabon national team.

In March 2019, Cousin formally signed a contract to serve as a coach of Gabon national team despite being confirmed in September 2018, but left a few weeks later before being appointed manager of French club ES Fos-sur-Mer.

[3] Cousin played in the lower leagues in France for Martigues and Niort before moving to Le Mans Union Club 72.

Lens qualified for the 2005–06 tournament after triumphing in the UEFA Intertoto Cup competition with Cousin starting in five games, making three substitute appearances and scoring three goals including one in the final itself versus CFR Cluj of Romania.

On 9 August 2007, Cousin arrived at Murray Park in Glasgow to hold contract talks over a move and signed a three-year deal for a reported fee of £750,000.

[7] In November 2007 he was linked with a transfer away from Rangers, and it was reported that he had a release clause in his current contract which means he could leave in January 2008.

[8] On 21 January 2008, it was reported that Premier League side Fulham had had a £2 million bid rejected by Rangers manager Walter Smith.

[9] The next day a bid of £3 million from Fulham activated a release clause,[10] however FIFA regulations meant Cousin required special dispensation to complete the move.

"[21] On 1 September 2008, Cousin signed for English Premier League side Hull City on a three-year deal for an undisclosed fee.

[39] In June 2020, Cousin joined the technical staff of FC Côte Bleue, where he would be in charge of the attacking section of the U18 to the senior reserve of the French club.