A late strike from 40 year-old midfielder Benjamin Nivet in Wednesday's (AEST) opening leg had given Troyes a one-goal advantage to take to Stade du Moustoir and it held firm to book a place in Ligue 1.
Benjamin Moukandjo struck the post in a tight first half, with neither side able to test the opposition goalkeeper before the interval.
Troyes goal-keeper Mamadou Samassa made a save from Arnold Mvuemba's free-kick and Jimmy Cabot and Moukandjo missed opportunities for the increasingly desperate hosts after the break.
The visitor - which won its last four games of the Ligue 2 season - survived a late barrage, leaving Lorient supporters to throw flares on to the pitch towards their dejected players at the final whistle.
Troyes joins Strasbourg and Amiens in Ligue 1, with Jean-Louis Garcia's men bouncing back to the top tier of French football after a single season in Ligue 2.