Warrenpoint took advantage of two Portadown errors to turn the tables on a first-half deficit and claim a memorable League Cup scalp at Shamrock Park.
Warrenpoint enjoyed early pressure, with Mark Clarke testing David Miskelly and Mark McCabe steering his header just past the target.
However, the Ports took the lead on 15 minutes thanks to Northern Ireland youth international Shea Conaty.
The teenager fired goalwards from distance and the shot bounced off the post, however, Conaty reacted first to race in and convert the rebound on his first start of the season.
Portadown finished the first half in control but without any additional reward, pushing Warrenpoint on to the backfoot thanks to a driving Ross Redman run plus clever skill between Peter McMahon and Mark McAllister.
The visitors got back on level terms eight minutes into the second half following a rare error by Miskelly, one of the Ports’ most consistent players.
The long-range free-kick was collected with ease by Miskelly - so often the Ports hero - but he failed to hold on to Nathan Murphy’s effort and the ball dropped over the line.
Portadown’s problems increased moments later when a foul by captain Keith O’Hara presented Markethill-based Stephen Murray with his spot-kick opportunity to grab the spotlight.
O’Hara came close to an equaliser but his looping back-post header on 73 minutes was cleared off the line by Roy Kierans.
Garry Breen headed past the post following a Redman cross off Darren Murray’s throw-in but Warrenpoint remained resolute.
It proved a night to remember for Warrenpoint - especially Murray, a former striker in the Mid-Ulster Football League with Portadown-based St Mary’s, plus Laurelvale’s Jordan Dane.
PORTADOWN: David Miskelly, Chris Casement, Keith O’Hara, Ross Redman, Garry Breen, Peter McMahon, Darren Murray, Mark McAllister, Tim Mouncey, Shea Conaty, Michael Gault. Subs: Billy Brennan, Ken Oman, Chris Ramsey (on for Mouncey, 58 mins), Marcio Soares (on for McAllister, 79 mins), Mark Carson.
WARRENPOINT TOWN: Aaron Shanahan, Nathan Murphy, Mark Clarke, Stephen Murray, Mark McCabe, Jordan Dane, Liam Bagnall, Stephen Moan, Roy Kierans, John Boyle, Sean Donnelly. Subs: Darren King (on for Clarke, 74 mins), Dermott McVeigh, Conor McDonald, Stephen Hughes (on for Bagnall, 58 mins), Mark Hughes (on for Murphy, 78 mins).
Referee: Ian McNabb.