Ballymena broke the deadlock inside four minutes when early pressure resulted in a goal for Kyle Owens.
Goalkeeper Chris McGaughey was handed a debut due to David Miskelly’s injury and he pulled off a smart full-length save to frustrate Jonny McMurray. However, the subsequent corner-kick was eventually delivered into the box by Tony Kane and Owens headed home.
A second was scored by Ballymena on 22 minutes when Danny Wallace and McMurray hustled to rob the ball off Brendan Shannon.
McMurray burst into the box from the left wing and managed to slide a shot beyond McGaughey.
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The visitors displayed the greater threat on the counter-attack and carved out openings for McMurray, Conor McCloskey, Cathair Friel and Caolan Loughran across the first half.
Portadown struggled to gain any sustained possession, with Nathaniel Ferris having a few attempts from distance.
The best chance for the home side arrived late in the half when Mark Carson’s long-range drive clipped the outside of Alan Blayney’s post.
The Ports’ second-half response featured greater energy but lacked a cutting edge.
One key effort of note arrived when Niall Henderson’s sweeping pass was directed forward by Shannon but Stephen Hughes’ shot on the run proved too high.
Another Hughes shot lacked power then substitute Mark McAllister created space but cleared the crossbar.
PORTADOWN: Chris McGaughey, Mark Carson, Adam Rodgers, Keith O’Hara, Alan Byrne, Brendan Shannon, Niall Henderson, Robert Garrett, Nathaniel Ferris, Stephen Hughes, Sam Simpson. Subs: Conor Larkin, Eoin Kirwan (Shannon, 65), Tim Mouncey, Mikey Withers (Simpson, 78), Mark McAllister (Ferris, 46).
BALLYMENA UNITED: Alan Blayney, Anthony Kane, Kyle Owens, Gary Thompson, Conor McCloskey, Cathair Friel, Danny Wallace, Jim Ervin, Leroy Millar, Jonny McMurray, Caolan Loughran. Subs: Ross Glendinning, Johnny Flynn, Allan Jenkins (Thompson, 61), William Faulkner (Friel, 77), Eoin Kane (McCloskey, 82).
Referee: Andrew Davey.