A first-half Darren Murray penalty on 21 minutes proved enough to give Portadown victory over Ballymena, who played much of the game with 10 men.
Murray scored his 15th goal of the season past makeshift goalkeeper Mark Surgenor after Stuart Addis had been sent-off for fouling Murray in the box.
Earlier, Surgenor had hit the post, David Cushley fired wide and Gary Breen blocked a Shane Dolan effort.
Tony Kane and Allan Jenkins had chances to equalise in the second half.
Kane's first-time shot from the edge of the box went just off-target, while Jenkins had a header saved and saw another headed effort hit the sidenetting.
The Ports had lost their two previous matches against the Sky Blues this season, suffering defeats in the Premiership and the League Cup quarter-finals at the Showgrounds.
The visitors had the better of the early chances, although Addis was forced into a save by Josh Barton, prior to the United stopper's dismissal.
Referee Keith Kennedy had no hesitation in pointing to the penalty spot and producing a red card after Murray brought down Murray in the area.
Gary Breen then had a shot blocked as Ronnie McFall's side looked to exploit their numerical advantage.
Ballymena had the better chances after the interval, but were unable to level despite their superiority.
Portadown remain fifth in the Premiership table, while Ballymena drop to ninth.