Horford was the third overall pick in the draft after helping Florida win consecutive NCAA titles.
The game tipped off at 10:30 a.m. for the Bobcats' annual "Cool School Field Trip," and the stands were packed with area eighth graders who earned an invitation by posting good grades and no absences since the start of the school year.
Down nine at halftime, Charlotte led three times in the third quarter, the last at 62-61 on a pair of free throws by Wallace with 3:15 left in the period.
The Bobcats tied it at 76 on a layup by Richardson with 10:35 remaining in the fourth quarter, but a jumper by Williams followed by Horford's dunk put Atlanta ahead to stay.
Atlanta center Zaza Pachulia left late in the third quarter after spraining his left knee.