Paul Connor's looping header gave the Stags hope but Danny Kedwell rounded Alan Marriott to make it 3-1 and RyanJackson raced through to fire a fourth.
But it was the Dons - who play a level above Fleet in non-league's top flight - who got their noses in front when Nwokeji reacted smartly to finish after Rashid Yussuff nodded Ryan Jackson's cross back across goal.