The central defender smashed home his second goal of the season from close range after goalkeeper Alex McCarthy could only parry substitute Sean Longstaff's 87th-minute piledriver.
However, the Saints will have headed off on the long journey back to the south coast feeling they deserved better on a day when home keeper Martin Dubravka had to make two fine first-half saves from Nathan Redmond.
Newcastle were sloppy in possession in the early stages as they struggled to cope with the visitors' high press, which allowed James Ward-Prowse to dictate from the middle of the field.
Shelvey had to bundle an inviting seventh-minute Ward-Prowse free-kick away from striker Ings at the near post with the Magpies searching in vain for a rhythm.
However, Allan Saint-Maximin sparked them into life three minutes later when he cheekily slipped the ball through full-back Cedric Soares' legs and carved his way into the penalty area before squaring for Miguel Almiron, whose shot was blocked.
Newcastle's carelessness almost cost them with 18 minutes played when Redmond ran on to Jetro Willems' loose pass and forced Dubravka into a vital one-one-one save which he bettered from the resulting corner to keep out the same man's rising strike.
The Magpies hit back with Saint-Maximin once again the instigator as he tricked his way past Pierre-Emile Hojbjerg and slid the ball into Joelinton's path, but the striker could not take it in his stride and Jack Stephens cleared.
Half-time came as something of a relief with the quality of the game having deteriorated alarmingly, and the home side returned knowing they had to be better.
Joelinton and Shelvey saw shots blocked in quick succession, but it was the visitors who took a 52nd-minute lead when Ings got in behind the home defence to latch on to Stephens' long ball and beat the advancing Dubravka.
The Slovakia international had to beat away Soares' shot two minutes later as the visitors threatened to kill off the game, prompting Steve Bruce to replace the struggling Joelinton with Carroll.
Carroll's presence gave Newcastle a different focal point and they gradually built up a head of steam, forcing Southampton to defend deep and in numbers.
And they got their reward 22 minutes from time when the striker drilled a fine cross to the far post for Shelvey to head home the equaliser.
But there was further drama in the closing stages when McCarthy parried Longstaff's shot to Fernandez and he fired gleefully into the roof of the net.