Angelo Balanta gave the hosts the lead on 13 minutes with a deflected strike from the edge of the box.
George Saunders added a second ten minutes later with a finish at the far post for the dominant Daggers.
But the Spireites dug deep and scored two goals in two minutes at the start of the second-half through Jamie Grimes and Kabongo Tshimanga to draw level.
Chesterfield set to announce signing of new goalkeeper ahead of Aldershot Town clash
'Active at both ends of market' - Chesterfield boss Paul Cook on latest transfer activity at Spireites
'It is quite simple' - Paul Cook reiterates Lucas Covolan warning after Port Vale loanee sees red on debut
Chesterfield striker could be loaned out to improve fitness after long time out
Chesterfield attempt to get defender's ban overturned before Aldershot Town match
Grimes was sent off late on for a second yellow card.
“I thought the start of the second-half was poor, two poor goals to concede,” McMahon said.
"We lost a marker in the box for the first goal and then for the second goal I will have to watch it back but it looks like we were too passive in terms of not picking up the best marksman in the division. We spoke about how good he is all week.
"I am disappointed but I thought in the first-half we were outstanding.
"We put a team together this morning – we had two late pull-outs last night – and the players who came in did excellent.
"It is frustrating because we were unbelievable in the first-half. Our football today was absolutely outstanding.”
Despite letting the lead slip McMahon expects his side to be challenging at the top once they get players back from injury.
"We have not had our whole squad fit this season,” he added.