A disastrous run of just one point from their previous four matches had raised serious question marks about United's ability to last the distance in the race for a top-two finish.
But two goals in three minutes on the stroke of half-time from Phil Jagielka and Paul Ifill effectively sealed the points for Neil Warnock's men.
Former Southampton striker Neil Shipperley netted with a clinical strike after 77 minutes to put the seal on only United's second victory in their last nine games.
Southampton lacked the stomach for a fight but it was they who engineered the first noteworthy goalscoring opportunity after six minutes.
Having collected possession 25 yards from goal, Kenwyne Jones cut in from the left flank and hit a fierce right-foot effort which forced Paddy Kenny into an excellent tip-over save.
With 19 minutes played United came agonisingly close to opening the scoring after Saints goalkeeper Paul Smith produced an outstanding save to deny Ifill.
The former Millwall winger weaved his way through a clutch of visiting defenders to give himself a clear sight of goal, but Smith's outstretched leg deflected Ifill's left-foot shot wide.
Ifill was in the thick of the action again moments later when he stole the ball from Southampton left-back Jim Brennan and unleashed a 14-yard shot which Claus Lundekvam blocked bravely.
But United struck the all-important opening blow after 43 minutes as Jagielka broke into the box to sweep home Chris Armstrong's low left-wing cross from six yards.
The left-foot finish was Jagielka's eighth goal of the season and further enhanced his reputation as one of the best players outside of the Premiership.
Ifill also has a keen eye for goal and two minutes later he drew level with Jagielka in the goalscoring stakes after also claiming his eighth of the campaign.
Rob Kozluk's long throw-in caused panic in the Southampton defence and when the ball fell to Ifill 14 yards from goal, the classy winger fired a low left-foot strike into the bottom corner.
The goals poured confidence into United and they came out after the break reinvigorated, with Ifill desperately unlucky not to claim his second after curling a delightful left-foot shot on to the top of the bar.
However, United's pressure told with 13 minutes remaining as Shipperley claimed the goal his work-rate deserved.
It came in bizarre circumstances as substitute Keith Gillespie headed Jagielka's cross towards goal, and Saints defender Brennan only succeeded in hitting his attempted clearance against team-mate Danny Higginbotham.
The ball then smashed the post and rebounded to Shipperley, who fired home a sweet left-footed strike from 12 yards to confirm an emphatic return to winning ways for the Blades.