Much the better side throughout, Rochdale were unlucky not to break the deadlock earlier when striker Paul Tait saw his close-range header brilliantly tipped onto the crossbar by Northampton keeper Lee Harper.
Lambert then struck 11 minutes after the break to settle matters, although Dale could have won by more.
Tait was unlucky in the latter stages with a volley that went just wide and a header that was hooked off the line by defender Lee Williamson.
At the other end Northampton, who are chasing an automatic promotion spot, failed to register a shot on target in a game in which they were totally outplayed.