Belmont, a No. 14 seed, led in the early going but Georgetown took control through the first half and held the Bruins off in the second period.
The Hoyas take a 24-8 record into a third-round game Sunday against 11th-seed North Carolina State, which upset sixth-seeded San Diego State 79-65 earlier Friday.
Belmont was clinging to an 8-7 early advantage but Georgetown scored 16 of the next 20 points for a 23-12 lead halfway through the first half and wasn't threatened the rest of the way.
The Hoyas shot 61.2 percent (30-of-49) for the game compared to 21-of-54 shooting (38.9 percent) for Belmont.
Clark hit 9-of-12 of his shots for his 21 points. Otto Porter finished with 16 and Henry Sims 15 for Georgetown. All nine Hoyas who played Friday scored at least two points.
Blake Jenkins, the only Bruins player to make more than half his shots, paced the Belmont attack with 17 points.