Media playback is not supported on this device.
Portadown jump over Ballymena and into seventh place in the table thanks to two second-half goals by Kevin Braniff.
Ballymena had led through a Chris Rodgers header in the eighth minute which went in off the underside of the bar after a Tony Kane cross.
However, Braniff equalised soon after the break with a fine finish from the edge of the box.
The Ports skipper then got on the end of Chris Casement's cross for the winner just after the hour.
That gave Ronnie McFall's only their second away win of the season and meant a fourth successive Premiership reverse for recent County Antrim Shield winners Ballymena.
The match was played at a high tempo from the start with Joe McNeill firing over for the Ports in the second minute.
James McCabe looked odds-on to put Ballymena into the lead but Ports keeper David Miskelly somehow kept out the strongly-hit shot and United skipper Allan Jenkins saw his follow-up effort deflected over.
But Ballymena soon forged ahead when Rodgers got on the end of Kane's cross from the right for the opener. It was the central defender's first goal for the club.
Portadown threatened to equalise straight away but Darren Murray's header was tipped over by home keeper Stuart Addis.
It could have been 2-0 though if McCabe had not fluffed a header in front of goal from Alan Teggart's left-flank delivery.
The miss was arguably the turning point of the contest as Braniff levelled on 50 minutes after being set up by McNeill.
Braniff's clinching goal came 11 minutes later as he produced a superb finish after Casement's cross.
The victory was only Portadown's second away win in this season's Premiership.