The 3-2 triumph at Tillysburn Park helped Harland and Wolff Welders extend the club's unbeaten league run to five fixtures.
It proved a day of high drama as set-piece play produced all five goals.
The afternoon also included a red card, second-half fightback, disallowed goal and penalty.
Scott McMillan broke the deadlock on 14 minutes with a close-range finish off a corner-kick delivery.
Manx Grand Prix: Michael Dunlop gears up for Classic Superbike victory bid on new Team Classic Suzuki GSX-R750
Organisers hope returning Mid-Antrim 150 will become permanent fixture on Irish road racing calendar again
‘Remember the sabbath day...’: Northern Ireland’s biggest Protestant church voices worries over Sunday football as Linfield gets ready to clash with Portadown
Kofi Balmer 'feels amazing' after making dream move to Crystal Palace
Linfield remain positive ahead of a massive fortnight for club
Portadown struggled to get back into contention and conceded a second in similar circumstances around the half-hour mark - with Dan McMurray steering home at a stretch from the edge of the box.
The Ports' dismal first half suffered a final setback with Niall Henderson's straight red card during the closing seconds due to an off-the-ball incident.
Portadown regrouped in the interval and produced a second-half display with greater drive and determination.
Kyle Neill's deadball ability helped Portadown win the Irish League title back in 2002.
Now in a second spell with his hometown club, with the goal of Championship promotion, Neill whipped in a corner-kick one minute after the restart which Adam McCallum forced home.
Another Neill corner-kick helped complete the comeback as McCallum nodded the ball across the face of goal and Kevin Amuneke's clever overhead hook left the sides level.
With the Welders on the backfoot, Portadown almost produced a dream turnaround - but a flag from a referee's assistant cut short the away celebrations after Amuneke's shot nestled in the net but Stefan Lavery was adjudged to have connected in an offside position.
Moments later came the decisive decision when referee Jamie Robinson felt Fra Brennan fouled Rainey inside the area and the veteran Welders forward slotted home the spot-kick.
Portadown had an injury-time header from Kyle McVey clear the crossbar but the final whistle sparked home delight and frustration in the away dugout at another loss due to self-inflicted errors.
H&W WELDERS: Robinson, Middleton, Armstrong, McMillan, McMurray, Harris, Deans, Nelson (Bonnes, 84), Gwiazda (Boyd, 66), Arthurs, Rainey (Withers, 88).
PORTADOWN: Connolly, Brennan, Neill, McVey, McCallum, C.Lavery (Kilmartin, 44), S.Lavery (Duffin, 77), Warwick, Amuneke (D.Henderson, 68).
Referee: Jamie Robinson.