Langston Hall added 15 points and Jakob Gollon 12 for Mercer (18-9, 11-3 Atlantic Sun Conference), which won its sixth straight game.
The teams exchanged baskets early on before Mercer broke open a 4-3 game with a 22-7 run. The Bears capped the run with four straight 3s -- two each by Gollon and Kevin Canevari -- and took a 27-13 advantage into intermission.
ETSU (13-14, 7-8) used an 18-3 second-half run to cut the deficit to 41-38 but the Bears answered the run with 10 straight points.
Mercer, which also defeated ETSU 78-59 on Jan. 17, made 12 of 25 shots from deep, with Smith and Hall accounting for eight.
Jarvis Jones led ETSU with 18 points and John Walton scored 12 before fouling out.