Niang placed his side on the path of victory by opening the scoring in the 16th minute of the game from the penalty spot.
In the 35th minute, Galtier’s men equalised through Loic Remy before Sarr restored the lead for the home team in the 59th minute.
Niang then completed his brace in the 71st minute after he was set up by Adrien Hunou to seal his side’s victory.
The 24-year-old attacker scored 14 goals in 44 appearances across all competitions this season.
Sarr featured for the duration of the game while Niang was replaced in the 89th minute by James Lea Siliki.
Cote d’Ivoire international Nicolas Pepe was introduced for Thiago Maia in the 73rd minute in an effort by Lille to avoid defeat.
With the win, Rennes ended the season in 10th spot with 52 points from 38 games.