Alonso, a freshman, was 5 of 9 from the field in his third career start. Marvin Smith and Kayel Locke added 13 points apiece for UNC-Greensboro (4-5), which plays its next five games on the road before hosting Furman on Jan. 5.
Deion Lane scored 13 points and Ray Chen had 12 to lead Belmont Abbey.
The Spartans closed the first half on a 20-9 run and led 40-24 at halftime. Alonso made two 3s during the stretch.
It was the first meeting between the schools.