Quincy Taylor hit back-to-back 3-pointers to cut Longwood's deficit to 75-73 with 22 seconds left. Jalen Carethers answered with a dunk, then Taylor missed a 3 and Radford made 3 of 4 free throws to seal it.
Radford (18-8, 9-4) moved into a four-way tie for first place atop the Big South Conference standings.
Rashun Davis had 13 points, Ya Ya Anderson 12, and R.J. Price and Brandon Holcomb added 11 apiece for the Highlanders.
Radford took a 44-33 lead into the intermission after shooting 58.6 percent, including 8 of 15 3-pointers.
Shaquille Johnson led Longwood (7-19, 3-10) with 22 points. Taylor added 16 and Leron Fisher 15. The Lancers opened the second half on a 10-2 run to pull within 46-43.
Longwood beat Radford in double overtime on Jan. 3.