Lambert struck twice in the second half to take his season's tally to 19 after the home side had apparently stolen the points with a convincing first-half display.
Northampton led in the sixth minute when, after having penalty appeals turned down by the referee when Gavin Johnson appeared to be held back by goalkeeper Matt Gilks, the ball ran free to Josh Low who coolly placed it in the corner of the net from 12 yards.
Northampton could have stretched their lead with a 25-yard volley by Martin Smith that caused Gilks some discomfort but they did double their lead ten minutes before the break.
The marksman was Scott McGleish, Northampton's top scorer who only a week earlier became the first Cobbler for 16 years to reach 20 goals in a season. His goal was a wonder strike with Eoin Jess delivering a corner which McGleish struck first time in blistering fashion as the ball cannoned into the net off the underside of the crossbar.
Rochdale could only point to a bad miss by teenager Gary Brown a minute after falling behind when he miscued a Ben Kitchen cross at the back post.
Dale had done little to suggest they could earn any reward but a double substitution at half time and an instant reply by Lambert in the 51st minute gave them fresh heart. The striker volleyed a stinging shot which Lee Harper in the home goal managed to get contact with, only to be beaten by the pace.
Northampton had chances to make the game safe with Johnson denied by Gilks in the 56th minute and then substitute Andy Kirk seeing his lofted effort 11 minutes from time drift just wide.
Lambert almost equalised with three minutes left when his volley whistled over before the injury-time drama.