The 29-year-old headed home deep into stoppage time to earn Saints a point that had looked unlikely when goalkeeper Bartosz Bialkowski gifted the visitors the lead.
Lambert had broken the deadlock for Southampton, before Chris Basham headed home and Callum McManaman's weak shot dribbled through the former Poland Under-21 international's legs and into the net. Lambert late equaliser helped save Bialkowski's blushes in part, while he will have been relieved to see second-place West Ham lose at Reading.
The Hammers were reduced to 10 men when Joey O'Brien was sent off, with Alex Pearce netting moments later before a Simon Church brace wrapped up the 3-0 win. Jack Collinson also received a red card late on.
Cardiff failed to close the gap on the top two after struggling to a goalless draw at Millwall, although Middlesbrough went level with the Welsh side thanks to Scott McDonald's goal in the 1-0 win over Brighton.
Hull moved up to fifth in the standings with another impressive away performance, with former Manchester United midfielder Corry Evans netting in a 1-0 win at Coventry, while Leeds retain the final play-off spot after Robert Snodgrass netted a penalty deep into stoppage time to cancel out Michael Kightly's opener.
Marlon King's brace helped Birmingham come from behind to secure a 2-1 win over Doncaster, who had taken a 50th-minute lead through Marc-Antoine Fortune.
David Norris' stoppage-time goal gave Portsmouth a 1-0 win at Burnley, while Glenn Murray netted the only goal as Crystal Palace secured an away win at Nottingham Forest.
Goals from Craig Bryson and Callum Ball helped Derby win 2-1 against Bristol City, who had taken the lead through Martyn Woolford, while Paul Gallagher's opener for Leicester was cancelled out by Lee Tomlin as Peterborough secured a 1-1 draw.
Source: PA
Source: PA