- Sean Longstaff (5', 21')
- Bruno Guimarães (82')
- Ché Adams (29')
Newcastle had won the first leg 1-0 away last week, with a late strike from Joelinton, and local man Sean Longstaff got them off to an ideal start in the return by opening the scoring in the fifth minute.
It was the midfielder's first goal at St James' Park in almost four years and he struck again in the 21st minute, knocking in a cut-back from Miguel Almiron to finish off a flowing team move sparked by a slalom run forward by Joe Willock.
Che Adams gave Southampton some hope with a bolt from the blue later in the first half after pouncing on a sloppy backpass by Willock and in the second half fellow striker Adam Armstrong had a glorious chance to level the score on the night but was thwarted by goalkeeper Nick Pope.
Newcastle midfielder Bruno Guimaraes was shown a straight red card in the 82nd minute after a VAR review for a studs-up tackle on Samuel Edozie but Eddie Howe's side survived playing the final stretch of the match with 10 men.
Under the competition rules, Guimaraes is set to serve a three-match ban in the league for the dismissal but will be free to play again in the cup final on Feb. 26.
It will be the Magpies' first final since they lost 2-0 to United in the FA Cup showpiece in 1999 and their second appearance in the League Cup final, having lost to Manchester City in 1976.
They will be bidding to win their first trophy since the 1955 FA Cup.
"I've never played in an atmosphere like that; we started off really well, scored two good goals, let them back into it a bit but we're happy to get it over the line," said defender Dan Burns.
"It's a special night, it's been a long time since we've had a night like this, an unbelievable night."
The win, achieved amid a scorching atmosphere and a sea of black-and white-scarves, was arguably the finest moment since Newcastle were taken over by a consortium backed by Saudi Arabia in October 2021, when the team were in the Premier League relegation zone.
They are now third in the standings and pushing for a return to Champions League football.
"It's amazing what can happen in a year, we would have laughed at you if you said this 12 months ago; it's really special what we're building at the moment," said Longstaff.
- Paul Tierney