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Magpies academy graduate Longstaff was the local hero at St James' Park, where his boyhood club were 2-1 winners in the second leg to seal a first cup final appearance since 1999.
The midfielder's first-half brace – and his first home goals in nearly four years – sent Eddie Howe's side to Wembley, though it finished the contest with 10 men after Bruno Guimaraes' late red card.
Che Adams scored a brilliant consolation for Southampton, which was denied a third final appearance in the competition.
The typically buoyant Toon Army were on their feet in the fifth minute when Longstaff exchanged passes with Kieran Trippier before sweeping the ball past Gavin Bazunu.
Longstaff went close again when he drilled just wide, but made no mistake in the 21st minute. A clever one-two with Joelinton released Joe Willock with the latter subsequently finding Miguel Almiron, whose inviting square ball was clinically tucked away by the midfielder.
Shell-shocked Southampton replied against the run of play eight minutes later. Willock's loose pass went straight to Adams, who denied the Magpies a ninth successive home clean sheet when he fired a stunning 25-yard effort into the bottom corner.
Saints almost clawed another goal back in the 73rd minute as Nick Pope denied former Newcastle striker Adam Armstrong.
Longstaff and Guimaraes went close to sealing it at the other end before the Brazil international received a straight red card following a VAR review for a late challenge on Samuel Edozie seven minutes from time.
But the numerical disadvantage could not deny the Magpies a long-awaited return to a Wembley final.
