The home side took the lead as early as the 10th minute when Wass was provided with the ball on the edge of the six-yard area before he converted the ball past Rudy Riou.
Wass came close to adding a second only to curl his free kick over the crossbar, but the bottom side did double their advantage shortly after the half-hour mark.
After a set piece wasn't cleared, John Mensah fired the ball into the net after managing to find a gap between defenders and goalkeeper.
Djakaridja Kone should have made it three for Evian before half time but after his header just evaded the target, Lens capitalised after the restart as Yoann Touzghar scored from close range after being set up by Jerome Le Moigne's header.
That encouraged the visitors, and three minutes after they had halved the deficit, they should have equalised when Deme N'Diaye received the ball 12 yards out but his effort was saved.
Fabien Camus could have put the result beyond doubt for Evian, but after he had seen a 20-yard shot saved, Lens pushed on in the latter stages.