Lehigh, which made it to the Football Championship Subdivision quarterfinals last season, took a 14-0 lead and led 21-7 at halftime before Monmouth battled back with 10 third-quarter points.
Monmouth drove to the Lehigh 10 in the fourth quarter, but the Mountain Hawks stopped Julian Hayes for no gain at their 7 and forced a fumble on fourth-and-1 with 7:12 remaining.
Lehigh, which has won its last 10 regular-season games, added a final touchdown on the last play when it recovered a Monmouth fumble after a completion and a couple of laterals.
Colvin, a senior, completed 17 of 32 for Lehigh with two interceptions. Ryan Spadola had nine catches for 111 yards after catching 96 passes a year ago.