A spectacular Rickie Lambert free-kick, his third goal in three home appearances since leaving Stockport, gave Dale a thoroughly deserved victory and lifted them to the edge of the play-off race.
Much the better side throughout, Dale were unlucky not to break the deadlock earlier when striker Paul Tait saw his close-range header brilliantly tipped onto the crossbar by Cobblers 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 could have moved within two points of automatic promotion spot, failed to register a shot on target in a game in which they were totally outplayed.
Southend booked their place in the LDV Vans Trophy final thanks to a 2-2 draw with Bristol Rovers in the southern final second-leg at Roots Hall.
They had won the first leg, at the Memorial Ground, 2-1, and will face Wrexham in the showpiece at the Millennium Stadium.
Chris Llewellyn netted to secure their northern final second-leg 1-0 win over Oldham, but the Latics were already down to ten men and staring defeat in the face when it happened.
His tame shot should have been no trouble for goalkeeper Les Pogliacomi but somehow the save went wrong and the ball trickled into the back of the net.
Wrexham had a two-goal lead going into the game, and a trip to Cardiff was never in doubt.