Harlequins fought back from an early 5-0 deficit to make it six wins out of six in the league this season, with tries from Sam Smith, Seb Stegmann and Joe Marler, combined with 12 points from the boot of Nick Evans, earning the visitors the spoils.
For Leicester, Boris Stankovich and replacement Scott Hamilton crossed for tries and fly-half Jeremy Staunton contributed the remaining eight points, but it was not enough to save their side.
The Leicester back will wonder how they lost the game when they had so much the better of the forward contest but the Tigers' backs were put to shame by the impressive counter-attacking efforts of their counterparts, who scored two delightful tries from scraps of possession
Saints' victory was never in doubt after opening up a 20-3 half-time lead courtesy of tries from centre James Downey and wing Paul Diggin. Both were created by Lamb, who finished with a 16-point haul by slotting two conversions and four penalties.
Wing Scott Armstrong rounded things off in the final moments with the hosts' third try.
Exeter, who had been third in the table entering this game, could only register an Ignacio Mieres penalty as they came off heavily second best at the breakdown as Saints' forward power proved too much to handle.
Sale held off a spirited fightback from Gloucester to claim their fourth win from six Aviva Premiership outings after edging a hard-fought encounter 13-11 at a rain-soaked Edgeley Park.
Sharks skipper David Seymour crashed over the line in the first half and a conversion and a penalty from fly-half Nick MacLeod helped the hosts into a 10-0 half-time lead.
But two penalties from Tim Taylor got Gloucester off the mark after the restart and a late try from centre Henry Trinder, which Taylor failed to convert, set up a grandstand finish.