Porter had 14 rebounds to complete the double-double and give the Hoyas (11-3, 1-2 Big East) their first conference victory of the season.
Georgetown's defense limited St. John's (9-7, 1-3) to 35.6 percent shooting from the field.
D'Angelo Harrison of St. John's, the leading scorer in the Big East with a 21.5-point per game average, was limited to seven points on 3-of-12 shooting.
The Hoyas won the rebound battle 42-28 and led 36-19 at halftime.
Jamal Branch led the Red Storm with 16 points.