After beginning his career with the Le Havre youth team, Niang turned professional at 18 years of age with Troyes. He had a mixed beginning to his career with his first season featuring ten Ligue 1 starts, all from the bench, the following season would show little improvement with 17 starts and only 3 league goals.

A loan period with second division Metz would allow him to hone his skills, contributing 5 goals in 12 appearances and helping Metz return to top flight french football for the 2003-04 season.

Jean Fernandez, the Metz manager, tried to make the loan deal into a permanent move but failed and Niang would leave Troyes for Strasbourg. The arrival of Danijel Ljuboja, to Strasbourg, at the same time would allow Niang to form an important strike partnership. At least until Ljuboja would move to Paris Saint-Germain. Niang would not score again for the remainder of the second half of that season.

The 2004-05 season would see Mickaël Pagis arrive at Strasbourg, and a fruitful new partnership would be formed, with the pair linking up for 27 goals in the league. This partnership would help Strasbourg make it to the Coupe de la Ligue final, where victory over Caen gave Strasbourg their second Coupe de la Ligue trophy.

In 2005, Niang would move to Marseille for a reported fee of € 7 million. He would be joined six months later by Mickaël Pagis. Niang finished the season as Marseille's top scorer with ten goals but Marseille would lose the Coupe de France final to Paris Saint-Germain.

Niang would go on to finish the 2007-08 season with 18 goals in Ligue 1, making him the second highest scorer in the league and Marseille's highest scorer with 28 goals in all competitions.

Niang was selected OM player of the month for May 2008.