Following a rather quiet first half, the home side's only chance arrived through Max Christiansen, who blasted high and wide of the target from an awkward angle after originally doing well to meet a ball in from Markus Suttner.
Ingolstadt were much improved in the second offering, and they went close through Benjamin Hubner who was denied by the post, while Lukas Hradecky had to stop strong efforts from Lukas Hinterseer and Max Christiansen.
The chances seemed to kick Frankfurt into gear and they soon came close twice through Aigner and Alexander Meier, the latter fluffing his lines when tasked with finding a way beyond Ramazan Ozcan.
Ingolstadt were finally able to turn their dominance into a lead when Gross put the hosts ahead in the 74th minute of the game.
The home side were then able to seal all three points with just under six minutes remaining when Lex scored the club's second to put an end to a potentially nervous finish.