Lorient had their first chance 10 minutes later, but Aboubakar volleyed wide after the cross from the right was played to him at a tricky height.
Aboubakar had another chance midway through the first half when he capitalised on a mistake in the Montpellier defence, but Geoffrey Jourdren was able to spare his teammates' blushes and make the vital stop.
Montpellier should have gone in front 10 minutes before the break when Remy Cabella floated in a free kick to the far post from 30 yards out, but Mounier, off balance, blazed over from all of three yards.
The game got its first goal just four minutes into the second half, as Barthelme controlled all over the top before taking it to the byline and striking past Jourdren and into the far corner.
Montpellier's night got even worse in the dying moments when they were reduced to 10 men. Hilton was shown a straight red card after appearing to kick Fabien Robert in the face on the floor after the two had collided.
There was still time for more controversy when Lorient were awarded a penalty in stoppage time and El Kaoutari was sent off for a mistimed last-ditch challenge from behind that brought down Aboubakar. The striker then picked himself up to slam home the second from 12 yards.
Lorient move to 12th following the victory, while Montpellier stay 17th.