The Bann started strongly but out-half Robin Thompson pushed an early penalty attempt wide of the posts. He made amends shortly afterwards with a well-struck effort to put his side 3-0 up.
Ballymena winger Caolan Fitzpatrick then capitalised on a loose ball and sent Alasdair Frazer under the posts for the first try of the game. Scrum half David O'Hara added the extras to put the Braidmen 7-3 ahead.
Banbridge fought back and Thompson kicked another penalty just before the break to leave the match in the balance at half-time.
The opening stages of the second period were nervy with neither team taking the initiative. A loose Banbridge kick in the 53rd minute was collected by Ulster Phoenix Academy full-back Ricky Andrew who launched a counter attack.
After the ball was recycled a couple of times, captain and man-of-the-match Paul Pritchard used his pace to touch down in the corner. O'Hara converted and added a penalty to bring the score to 17-6.
Banbridge rallied late on and replacement second row Ryan Patterson forced his way over with five minutes left to give his side hope.
They tried in vain to add another score but Ballymena closed the game out to end the calendar year as league leaders and Ulster Senior champions.