Aaron Wilbraham hammered the final nails in Northampton's relegation coffin as Stockport banished any remote fears of the drop themselves.
Wilbraham took his recent tally to five goals in three games, the young striker proving a revelation in only his fifth start of the season after a long-term injury.
Northampton will be relegated over the weekend for sure if Barnsley and Chesterfield get wins, but the way they fell 3-0 down inside 24 minutes confirms their fight is at an end.
County, only beaten at Crewe in their last seven games, were quickly on the march to a third straight win.
The game's first chance in the seventh minute produced their opener when Dave Challinor's long throw penetrated deep into the home box and Wilbraham scored with a header which Jerry Gill just failed to clear from behind the line.
Challinor almost made it 2-0 minutes later, but Jim Goodwin did just that in the 14th minute when he curled a precision right-foot free-kick from the edge of the box inside Lee Harper's left-hand post.
The game was dead and buried in the 24th minute when Goodwin crossed from the right and Wilbraham rose to head past Harper.
From that point onwards, Northampton had most of the play and the majority of the chances but County had done their job and comfortably snuffed out the home threat.
Marco Gabbiadini fired wide from the edge of the box on the half hour and after the break should have done better when miscuing his shot with the goal gaping in front of him.
Midfielder Paul Harsley worked hard and could have reduced the arrears with a 69th minute free-kick which scraped the woodwork, while right at the death, substitute Derek Asamoah lofted a shot onto the roof of the net.