But they were made to fight for victory as manager Colin Calderwood's former side did their best to put a dent in their title hopes.
And it took a well-taken header from Julian Bennett in the second half to earn the Reds a 1-0 success and bring an end to a stubborn resistance that had been led by Cobblers goalkeeper Mark Bunn.
It was a scrappy affair early on and it took until the tenth minute before either team carved out a chance of any merit with Grant Holt guiding a volley wide of the post after being picked out well by a deep cross from Bennett.
David Prutton again looked a lively figure for the home side and was the catalyst in another decent move as he left Ian Taylor for dead on the right of the box and crossed dangerously but to no avail.
Prutton was even more unlucky after striking a sweet 25-yard drive that Bunn did superbly to push on to his post and out for a corner.
It was to be the first of three quick flags-kicks for the Reds and they came close to scoring from two of them as Wes Morgan saw a header cleared off the line by Kenny Deuchar and Scott Dobie's powerful downward effort was deflected narrowly over the bar.
Grant Holt also worked keeper Bunn with a 30-yard drive that he dealt with capably, while Dobie headed another effort wide.
But it was the visitors who finished the half with a flourish and who should have scored.
Firstly Deuchar planted header right at Paul Smith when presented with a good opportunity, then Taylor saw a powerful close-range effort cannon back off the post and away to safety.
Forest restored their dominance after the break but still couldn't find the breakthrough, with Ian Breckin seeing Bunn save after Dobie had beaten the keeper to a high ball and Dobie having a close-range volley blocked.
And Bunn had to come to Northampton's rescue again as Jack Lester showed neat feet to engineer room for a shot which looked certain to arrow into the roof of the net from eight yards before the keeper somehow stretched out an arm to push over.
Sam Aiston twice hit back for Northampton and saw a shot blocked by Morgan in the box before firing another long-range effort wide.
But Forest's persistence was finally to pay off as, after a handball from Andy Holt close to the corner flag, they earned themselves a free-kick. James Perch fired in an inch-perfect cross for Bennett to head powerfully home in the 66th minute.
Aiston came close to bringing Northampton back on level terms with a well-struck low shot that flew narrowly wide, but otherwise the visitors never looked likely to find a way back.