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 allowed 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 left Troyes for Strasbourg. The arrival of Danijel Ljuboja, to Strasbourg at the same time allowed Niang to form an important strike partnership until Ljuboja moved to Paris Saint-Germain. Niang didn't score again for the remainder of the second half of that season.
The 2004–05 season saw Mickaël Pagis arrive at Strasbourg, and a fruitful new partnership was formed, with the pair linking up for 27 goals in the league. This partnership helped 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 moved to Marseille for a reported fee of €7 million. He was joined six months later by Mickaël Pagis. Niang finished the season as Marseille's top scorer with ten goals but Marseille lost the Coupe de France final to Paris Saint-Germain.
Niang would go on to finish the 2009–10 season with 18 goals in Ligue 1, making him the 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.
He signed a 3+1 year deal with Fenerbahçe on 14 August 2010 for a fee of reported €8 million. Mamadou Niang had made a great start with Fenerbahçe SK when he scored 7 goals in his first 6 league appearances. Niang scored his first hat-trick for Fenerbahçe on his 5th appearance when they won 6–2 against Kasımpaşa. Niang helped his side win the 2010–11 Süper Lig, scoring 16 goals in 29 appearances.
On 6 September 2011 he was sold to Al-Sadd for €7.5 million.
On 19 October 2011, at the 2011 AFC Champions League Semi-final match against Suwon Samsung Bluewings, he scored the controversial, un-sportsmanlike second goal for his team. Al Sadd should've sent the ball back to the Bluewings since the play was suspended when the home team's player was injured. However, while Suwon's defense stood still thinking their goalkeeper will get the ball back, Niang stole the ball and went past the goalkeeper to score. The goal induced an intrusion of a home fan and a huge melee of both players and bench. The controversy got bigger when Al Sadd's player Keita took a swing at a home fan. Even though his goal was against general notion of fair-play, Niang argued that there was no problem with his goal, and that it was Suwon who played without manners, inducing melee between two teams. After all, AFC did not conclude this issue justly, by giving disciplinary actions to Suwon only, Al Sadd getting absolutely nothing, not even Niang or Keita. He then got himself sent-off in injury time for kicking the ball away when he was flagged offside, receiving another yellow card in addition to his previous, meaning he could not participate in the second leg in Doha.
On 31 January 2013, he moved to Turkish side Beşiktaş on loan until the end of the season.
On 3 March 2013, he scored his first goal and contributed with an assist in a 3–2 win against his old club Fenerbahçe securing 3 points for his team in the final derby ever on İnönü Stadium.
On 28 August 2014, Niang returned to France to sign for AC Arles-Avignon, having been overseas for the last four years.As of 16 September 2011.
ES Troyes ACUEFA Intertoto Cup: 2001
RC StrasbourgCoupe de la Ligue: 2005
Olympique de MarseilleLigue 1: 2009–10
Coupe de la Ligue: 2010
Trophée des Champions: 2010
UEFA Intertoto Cup: 2005
FenerbahçeSüper Lig: 2010–11
Al SaddAsian Champions League: 2011
UNFP Player of the Month (3) : May 2005, December 2005, December 2007
Ligue 1 Team of the Year (2) : 2007–08, 2009–10
Ligue 1 Goal of the Year (1) : 2009–10
Ligue 1 top goalscorer (1) : 2009–10