Longstaff picked the perfect time to find the target for the first time since October, firing home twice in the opening 20 minutes to hand Eddie Howe's men a 3-0 lead overall in their two-legged semi-final.
Che Adams pulled one back for the Saints before the break but they couldn't muster a comeback, despite the hosts being reduced to ten men late on when Bruno Guimarães was dismissed for a challenge on substitute Sam Edozie.
And the Magpies stood firm despite their playmaker's red card, sending a sold-out St. James' Park crowd wild as they made sure of their spot in February's final under the arch in the capital, with a first domestic trophy in 68 years now tantalisingly within reach.
Holding a one-goal lead from the first leg at St Mary's, United were quick out of the traps in front of their own supporters. Callum Wilson stabbed a Longstaff cutback wide early on but it took just five minutes for the hosts to find a way through and calm the nerves of those of a black and white persuasion. It owed much to Guimarães' skill in midfield as the Brazilian found Longstaff, who in turn picked out Trippier on the right. The full back cut inside and found Longstaff unmarked in the area and he took a touch before picking out the bottom corner in front of the Gallowgate End.
The goal deflated Southampton, who struggled in the minutes immediately after the opener. Longstaff almost made it two, blasting just wide with his left after more craft from Guimarães, while Joelinton saw an effort blocked, but with 20 minutes gone Howe's side cut through the Saints once again. It began near halfway, with Joe Willock playing a one-two with Joelinton and marauding down the left. He played in the overlapping Miguel Almirón and the Paraguayan's low ball across the box was perfect for Longstaff, who again arrived in the area at just the right time to coolly roll home his second of the night.
But just before the half hour, the visitors found a response. Willock's misplaced pass surrendered possession and Adams picked the ball up 25 yards from goal, with the Scotland international opting to take a shot early. It was well struck and fizzed beyond Nick Pope, ending the Magpies stopper's record of 15 consecutive clean sheets.
Saints - who had already lost Kyle Walker-Peters to injury - introduced Romain Perraud and Romeo Lavia at the break in a bid to re-energise their midfield and began the second half with renewed purpose, as Newcastle tried to regain the level of dominance they had shown before the restart.
Howe made some switches of his own, bringing on Allan Saint-Maximin, Alexander Isak and Jacob Murphy, but the fluency that marked their first half display proved elusive as the contest became fragmented and scrappy. Adam Armstrong, the former Magpie leading the line for Southampton, squandered a great opening when he latched on to Lavia's through ball and tried to dink his finish over Pope, but the former Burnley man saved with his legs.
At the other end, Longstaff's rasping drive had Gavin Bazunu scrambling to his right to palm it away and deny him a first senior hat-trick, before Guimarães' left footed curler grazed the post with just over ten minutes remaining - just a few minutes before his challenge on Edozie saw him receive a straight red card after referee Paul Tierney reviewed the incident on the pitchside monitor.
But in the end the visitors couldn't capitalise on their brief numerical advantage as Howe's charges saw out the final stages and St. James' erupted at the end of an unforgettable night for Newcastle supporters - who can now book their trips to London for next month's final.
