With regular number one Nathan Bishop ruled out injured for the game, Wombles boss Johnnie Jackson called on McDonnell to make his first league start for the club since 2019 - with no recognised goalkeeper named among the substitutes.
However, the former Eastleigh stopper was largely a bystander as Wimbledon totally controlled the first half.
Their early dominance was rewarded after just 10 minutes.
James Tilley's cross was nodded down by Steve Seddon and looked to be heading out harmlessly for a goal-kick.
But Browne chased the ball down and played a brilliant backheel back into danger, with Maycock finishing first-time at the near post.
Tilley was creating havoc on the Cobblers' left-hand side and his curling cross somehow sneaked between the boot of the diving Matty Stevens and the back post, before drawing a good save from Lee Burges after shooting through a sea of bodies.
Northampton boss Nolan had seen enough and looked to his bench on 31 minutes, bringing on an extra defender in Max Dyche for Tyrese Fornah and reverting to a back three.
Burge made another fine stop to deny Maycock a second on 45 minutes but it was a different game after the break.
Northampton's 17-year-old forward Evans, on loan from Leicester City, drew two fine stops from McDowell - firstly down low to his right and then an unorthodox save with his feet - before again testing the Wimbledon keeper as the Cobblers looked a team transformed.
It looked like it might be a familiar story for Wimbledon - no team in League One has conceded more late goals - but ultimately they were able to see the game out with a clean sheet, though lost Tilley to injury in the second half.
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