However, that lead lasted for just four minutes as Carlos Sanchez equalised with a headed goal from close range.
It became 2-1 to Valenciennes in the 14th minute when Anthony Le Tallec shot home from the centre of the Lorient penalty box with his left foot.
Things went from bad to worse for the visitors minutes later when Arnaud Le Lan was dismissed for a bad challenge.
Lorient were reduced to nine men in the 76th minute when Innocent Emeghara was sent off and conceded thrice in the match thereafter.
Vincent Aboubakar was sent through by Le Tallec for his first goal on 81 minutes before Le Tallec added his second six minutes later.
Aboubakar rounded up proceedings at Stade du Hainaut in the final minute of normal time with his second goal of the evening.