Dagenham assistant manager Wayne Burnett was left cursing an outstanding save from Northampton goalkeeper Chris Dunn which helped to secure a 1-0 victory for the Cobblers.
Full-back John Johnson slammed home the winner in the 71st minute with a glorious strike, before Dunn made an unbelievable save from Peter Gain just moments afterwards.
Burnett said: "We created a few chances, but their keeper has pulled off some worldie saves.
"He also made some great stops from Jon Nurse and Damien McCrory too. On another day they may have gone in, but it didn't go for us.
"They scored a fantastic goal too, the full-back has come inside and bent one into the top corner.
"There wasn't a great deal of quality in the game and you got the feeling that whoever got the first goal would turn out to be the winners and that's how it happened."