Lens have won all seven home games so far this campaign but missed the chance to open the scoring in the first half when Seko Fofana's 38th minute penalty was saved by Toulouse goalkeeper Maxime Dupe.
They took the lead on the hour mark when Openda got in front of his marker and headed in from a Florian Sotoca cross. Both Openda and Sotoca had come on as substitutes just four minutes earlier.
Openda scored again in the 86th minute when he got past the Toulouse defence and flicked the ball beyond Dupe with his left foot.
The 22-year-old Belgian completed his hat-trick two minutes into stoppage time with a shot from the right side of the area that went through the legs of Toulouse defender Moussa Diarra and into the left corner of the net.
Lens are in second place in the standings with 30 points from 13 games. They are two points behind leaders Paris St Germain, who play 11th placed Troyes on Saturday.