Aston Villa slipped into the bottom three in controversial circumstances as Jay Rodriguez's blatant first-half dive helped Southampton to a valuable win.
Villa rallied after the break and went closest to an equaliser when Nathan Baker headed onto the crossbar.
The result saw the Saints leapfrog Villa and move clear of the drop zone.
The winter months have not been kind to Villa's inexperienced side, and the embarrassing midweek League Cup semi-final first-leg defeat to Bradford was their fourth in seven games in all competitions.
While Villa's slide down the Premier League table had seen them take only 10 points from their last 11 fixtures, Nigel Adkins's ever-improving Southampton outfit came to Villa Park on the back of only two losses in 10 league games.
Despite the disparity in recent form, Villa started the brighter and forced a number of corners as giant centre-forward Christian Benteke repeatedly pressured the slight figure of visiting right-back Nathaniel Clyne.
Villa's problems have been exacerbated by a long dry spell in front of goal - they had only scored seven times in their last 10 league matches - and again they were unable to turn momentum into chances.
Southampton's superior confidence on the ball then started to show and Rodriguez should have done better after he was smartly played into space on the right wing by Lambert but his control at pace let him down in the area and he could only scuff a weak pass into goalkeeper Brad Guzman's arms.
Referee Mark Halsey had earlier angered the Southampton bench by failing to punish a high challenge from Stevens on Gaston Ramirez and a flailing elbow from Benteke on Maya Yoshida, but the game's key officiating decision came in the 33rd minute when he pointed to the spot after Rodriguez tumbled in the area.
While the furious Villa players raced to question Halsey's call, Rodriguez attempted to plead his innocence by mouthing: "I did not dive."
Lambert, who joined joined Southampton for £1m in 2009, then stepped up to send Guzan the wrong way and retain his perfect penalty record for the club.
Villa manager Paul Lambert took off the ineffectual Brett Holman at half-time and switched to a 4-4-2 formation, and his decision led to a much improved performance.
Benteke should have scored his 10th goal of the season after he picked up on goalkeeper Artur Boruc's dangerous parry from Stevens' 25-yard effort but the Belgium international could only side-foot his effort over the crossbar.
The previously invisible Charles N'Zogbia then struck a firm low drive into the Pole's arms, though Southampton's dangerous attacking options came to the fore once again as the exciting Jason Puncheon struck the base of the post after cutting inside from the right.
Villa continued to push for a vital leveller and again Benteke was the man at the centre of things. First, he smartly twisted into space on the edge of the area before firing a wild effort wide with his left foot , then a more measured curling effort with his favoured right went inches beyond the post.
A desperate late rally went to no avail as Baker hit the woodwork for Villa before Benteke and substitute Jordan Bowery failed to find the target with close-range efforts.
While Wigan's draw at Fulham pushed Villa into the bottom-three, Southampton's resilience was rewarded with a fifth win in six games against their fellow strugglers.