At PAT stadium, Muang Thong were dominant throughout.
Piyaphon Buntao scored two goals in less than two minutes in the second half, while striker Christian Kouakou erased any doubt with a left-footed shot in the 89th minute.
The showdown got off to a wet start in this raucous affair, and yellow cards and uneven play were the order in the first half as both sides returned from a long mid-season lay-off.
The Kirins started to enjoy more possession near the end of the first half after Port were reduded to 10 men after 38 minutes.
Siwakorn Jaksuprasart was dismissed after a second bookable offence when he tripped Jakkraphan Pornsai.
Piyaphon broke through in the 52nd minute with a low drive past third-string goalkeeper Wanlop sae Jiew.
Barely a minute later, the two-time defending champions broke down the left side and rocketed a shot off the keeper, and Piyaphon was able to head the ricochet past the fallen keeper for a 2-0 advantage.
The Kirins kept the pressure on and five minutes later Jakkraphan made a mazy run dribbling past several defenders to reach the box before blasting over the bar.
A minute later Christian laid off what seemed to be an easy hat-trick for Piyaphon but he fired wide.
Former England and Liverpool striker Robbie Fowler entered the match after 61 minutes to much applause, but was unable to ripple the cords before Christian's 20-yard effort just before the final whistle.