Mikael Hopkins, who added 13 points, hit a layup to start the second half, and after the Mountaineers' Xavier Owens scored to make it 36-34, the Hoyas (4-1) scored eight straight -- including four by Hopkins -- to lead 44-34.
The Mountaineers (1-3), who missed their first five 3-pointers of the second half, drew to within 50-44 on Rashad Whack's 3-pointer with 8:22 remaining. Whack led Mount St. Mary's with 11.
Georgetown quickly answered with eight points -- four by Porter -- to lead 58-44 with 4:13 left and finished the game on a 22-6 run.
The Hoyas had a 42-21 rebounding advantage and held Mount St. Mary's to 25.8 percent second-half shooting (8 of 31).
Mount St. Mary's, under first-year coach Jamion Christian, played Georgetown well for the first half. Taylor Danaher's jump tied the score at 23-all with 5:29 to play before halftime.
Georgetown scored six straight to take a 29-23 lead, but Sam Prescott's 3-pointer pulled the Mountaineers to within 34-32 at halftime.
The Hoyas hit 63.6 percent (14 of 22) from the field in the first half but had 11 turnovers.
Georgetown is waiting to see if it will enter the Top 25 Monday. The Hoyas beat 11th-ranked UCLA last Monday in the Legends Classic and lost by four to No. 1 Indiana the next night.