The Magpies had to play for over an hour with 10 men after Fabian Schar was sent off in the 28th minute for violent conduct.
Schar saw red for headbutting Saints striker Ben Brereton Diaz.
Southampton had earlier seen a Jack Stephens goal disallowed for offside.
But Newcastle took the lead despite their numerical disadvantage just before half-time when Joelinton punished an error by goalkeeper Alex McCarthy.
More to follow.
Newcastle took the lead despite bring down to 10 men after Fabian Schar (left) had been sent off in the 28th minute for headbutting Ben Brereton Diaz (right)