Lambert drove home from 15 yards at St Mary's, four minutes before the break on Tuesday night after Graeme Murty and Papa Waigo combined to set up the deadly marksman.
Saints should have had the result done and dusted on countless opportunities after half-time with Lambert, Adam Lallana and substitute Michail Antonio all guilty in front of goal.
After a dull opening 20 minutes, Alan Pardew's side breathed some life into the game as Lallana's skill took him into space but the much-coveted midfielder could not find the target.
Waigo's candidate for miss of the season left the home crowd frustrated as the Senegalese striker cleared the crossbar from just three yards out.
With Gary Waddock's men on the ropes, Lambert then delivered the fatal blow as Murty found Waigo in space for the on-loan striker to square to his partner in crime.
Wycombe struggled to create much after the interval and had goalkeeper Scott Shearer to thank for keeping the scoreline to a minimum as he denied Lambert and Lallana with a fine double save.