Dave Clarke’s stunning solo score secured the Qualifying 1 points for Ballyclare on a cold afternoon at The Cloughan.
Portadown had been ahead for most of the game, but going in to the final quarter they held a slender one-point advantage, but that was wiped out when Clarke sprinted clear to seal a fine 18-12 win.
Robert Smith opened the scoring with a penalty, but soon the Ports were ahead with a converted try.
Despite having had territorial advantage Ballyclare turned round trailing 6-7, Smith having added a second penalty.
Portadown had the edge after the interval and increased their lead to 12-6 with an unconverted score.
Having weathered the storm, scrum-half Colin Morrow threw his side a lifeline when he nipped over on the blind side for a score that Smith was unable to improve upon.
With the match finely balanced at 11-12, the hosts snatched the points when Clarke broke through the first tackle at half-way, and sprinted clear for the winning score that Smith goaled.
Coach Chris Campbell said: “We were on top in the first-half, but failed to convert possession into points, whereas Portadown had one scoring opportunity and took it.
“With the elements in their favour in the second-half they had a purple patch of 25 minutes, but our defence was superb, and then late scores by Colin Morrow and Dave Clarke saw us through.
“Skipper Robert Boyd was tremendous in that tense finish, when his strong leadership ensured that we held our nerve.”
The win keeps Ballyclare in fourth position in their first season in Qualifying 1.