Germain showed his teammate how to remain calm in front of goal moments later as he beat the offside trap before slotting past a helpless Alphonse Areola.
Bastia responded well after suffering the early setback, and Ryad Boudebouz came close to responding immediately with a powerful free kick which forced Danijel Subasic into a good save.
An extended spell of Bastia dominance was rewarded midway through the first half when Maboulou found space to power a header home from Francois Kamano's cross.
Bastia were denied a second goal by Subasic, who displayed impressive reflexes to keep out Sebastien Squillaci's header following good work from Boudebouz.
Lacina Traore, back in the Monaco side as a replacement for the injured Dimitar Berbatov, should have done more to capitalise on a rare opening before the break, but he could only slice wide while under pressure in the box.