Andrew Boyce's late equaliser had cancelled out Sido Jombati's delicate free-kick, but the home earned all three points in stoppage time.
Wycombe defender Joe Jacobson's free-kick on 13 minutes drew a decent save from James McKeown.
Garry Thompson had another good chance for the hosts on 50 minutes, but wastefully blazed his half-volley wide from 25 yards.
Wycombe led five minutes later, though. Luke Summerfield brought down Jacobson on the edge of the box, and Jombati stepped up to place the ball beyond McKeown for the home side's first goal in 112 days.
Cameron Dawson was forced into a sharp double save after 69 minutes, diving to his right to stop Summerfield's fierce shot, then saving Kayden Jackson's follow-up with his legs.
Ten minutes later, though, Grimsby deservedly equalised when Boyce passed the ball beyond Dawson after Wycombe failed to clear a corner.
But Stewart popped up to tap home from two yards after substitute Adebayo Akinfenwa's flick-on bounced off both posts for Wycombe's first points of the season.
Report supplied by the Press Association.
Match ends, Wycombe Wanderers 2, Grimsby Town 1.
Second Half ends, Wycombe Wanderers 2, Grimsby Town 1.
Joe Jacobson (Wycombe Wanderers) wins a free kick in the defensive half.
Foul by Kayden Jackson (Grimsby Town).
Attempt blocked. Ashley Chambers (Grimsby Town) right footed shot from the centre of the box is blocked.
Goal!  Wycombe Wanderers 2, Grimsby Town 1. Anthony Stewart (Wycombe Wanderers) left footed shot from very close range to the centre of the goal.
Adebayo Akinfenwa (Wycombe Wanderers) hits the left post with a header from the centre of the box.
Substitution, Grimsby Town. Rhys Browne replaces Tom Bolarinwa.
James Berrett (Grimsby Town) is shown the yellow card for a bad foul.
Joe Jacobson (Wycombe Wanderers) wins a free kick in the defensive half.
Foul by James Berrett (Grimsby Town).
Substitution, Wycombe Wanderers. Nick Freeman replaces Matt Bloomfield.
Corner,  Wycombe Wanderers. Conceded by James McKeown.
Attempt saved. Garry Thompson (Wycombe Wanderers) right footed shot from the right side of the box is saved in the bottom right corner.
Sam Wood (Wycombe Wanderers) wins a free kick in the defensive half.
Foul by Kayden Jackson (Grimsby Town).
Foul by Matt Bloomfield (Wycombe Wanderers).
James Berrett (Grimsby Town) wins a free kick in the attacking half.
Attempt missed. Sam Wood (Wycombe Wanderers) left footed shot from outside the box misses to the left.
Goal!  Wycombe Wanderers 1, Grimsby Town 1. Andrew Boyce (Grimsby Town) left footed shot from the left side of the box to the bottom right corner following a corner.
Corner,  Grimsby Town. Conceded by Aaron Pierre.
Attempt blocked. Kayden Jackson (Grimsby Town) right footed shot from the centre of the box is blocked.
Attempt saved. Omar Bogle (Grimsby Town) header from the centre of the box is saved in the top centre of the goal.
Corner,  Grimsby Town. Conceded by Anthony Stewart.
Corner,  Grimsby Town. Conceded by Dan Rowe.
Attempt blocked. Omar Bogle (Grimsby Town) left footed shot from outside the box is blocked.
Substitution, Grimsby Town. Ashley Chambers replaces Sean McAllister.
Substitution, Wycombe Wanderers. Dan Rowe replaces Stephen McGinn.
Foul by Adebayo Akinfenwa (Wycombe Wanderers).
Luke Summerfield (Grimsby Town) wins a free kick on the right wing.
Anthony Stewart (Wycombe Wanderers) wins a free kick in the attacking half.
Foul by Kayden Jackson (Grimsby Town).
Attempt missed. Tom Bolarinwa (Grimsby Town) right footed shot from the centre of the box is close, but misses to the right.
Foul by Matt Bloomfield (Wycombe Wanderers).
James Berrett (Grimsby Town) wins a free kick in the defensive half.
Substitution, Grimsby Town. Omar Bogle replaces Scott Vernon.
Attempt missed. Tom Bolarinwa (Grimsby Town) header from the centre of the box is too high.
Attempt saved. Josh Gowling (Grimsby Town) left footed shot from outside the box is saved in the top centre of the goal.
Substitution, Wycombe Wanderers. Adebayo Akinfenwa replaces Dayle Southwell.
Hand ball by Matt Bloomfield (Wycombe Wanderers).
Anthony Stewart scored an injury-time winner against Grimsby to earn Wycombe Wanderers their first victory since March.