MK Dons came close to pulling off a dramatic comeback at Sixfields but they weren't able to fully recover from three early Northampton Town goals.
Harry Beautyman, Alex Revell and Matt Taylor had the Cobblers three-up inside the opening half an hour but Dean Bowditch and Samir Carruthers both found the net for the Dons to reduce the deficit.
However, Karl Robinsn's men didn't leave themselves enough time to grab an equaliser and, as a result, fell to their first away league defeat of the campaign.
Boss Karl Robinson included Deadline Day signing Ryan Colclough in his starting XI, while he was also able to include Kieran Agard in his match day squad after his recovery from a calf injury.
It was Northampton who opened the scoring on eight minutes when confusion at the back allowed Beautyman to nip in and shoot across Martin into the bottom left-hand corner.
The Cobblers then doubled their lead through former Dons striker Revell’s header, before things went from bad to worse when Taylor made it three with a well-placed free-kick on 28 minutes.
To say it was a far from ideal start would be an understatement but with nothing to lose, the Dons went in search of a goal that would get them back into the game.
Nicky Maynard produced a save out of Adam Smith, with Ed Upson also going close soon after - a 25-yard effort which fell just past the post.
Their response was eventually rewarded in the 38th minute when Bowditch climbed well at the far post to head home Samir Carruthers’ cross.
The Dons almost gave themselves an added boost ahead of the break when Colclough worked his way into the Cobblers’ penalty area however his low ball across the face of goal wasn’t met by a teammate.
Robinson couldn’t have been best pleased with his players at the break and perhaps it was his stern words which led to their fast start to the second period.
Carruthers had a left-footed shot blocked on 50 minutes before Potter struck a first-time half-volley wide from Colclough’s pass.
The Dons’ chances of pulling off a dramatic comeback appeared to fade away, though, on 56 minutes when Northampton were awarded a soft penalty for a foul on Kenji Gorre by Bowditch. Taylor took the spot-kick for the hosts but was denied by a terrific save from Martin.
Robinson introduced Kieran Agard and Ben Reeves just before the hour mark to give his side an attacking boost and they almost grabbed a second goal on 67 minutes when Baldock’s drilled effort came close to catching out Smith in the Cobblers’ goal.
Carruthers and Reeves then both had shots blocked soon after before the former finally found a way past Smith with a fierce volley.
The goal, though, came in the fourth of four additional minutes and despite their best efforts Robinson's men weren't able to find that a third.
Starting XI: Martin; Baldock, Downing, Walsh, Lewington (c); Potter, Upson (Reeves 59); Carruthers, Bowditch, Colclough (Powell 71); Maynard (Agard 58).
Unused subs: Nicholls, GB Williams, Hendry, Thomas-Asante.
Bookings: Carruthers, Baldock
Goals: Bowditch (38), Carruthers (90+4)
Attendance: 6,618 (1,269 away)