Rochdale were coasting until they had to play out the last 20 minutes with ten men after substitute Lee Thorpe was sent off three minutes after he came on.
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Rotherham forfeited a free-kick 25 yards out as Rochdale turned on the pressure and Clark Keltie decided on a cheeky quick effort as the home side were still trying to organise a wall, but keeper Andy Warrington gathered it smartly.
Another spell of pressure saw Adam Le Fondre hit a snapshot that failed to trouble Warrington before a Simon Ramsden cross saw Jordan Rhodes go close with a header.
Le Fondre produced a shot on the turn that hit Ian Sharps, before Adam Rundle made a weaving run through three challenges but his tame shot was straight at Warrington.
Rotherham's had two efforts from distance when Reid's fierce drive had keeper Sam Russell struggling and Russell had to produce another save when Danny Harrison slammed another testing effort.
But four minutes before the break, Rochdale made the breakthrough.
Warrington managed to parry a goalbound shot from Will Buckley but Rhodes reacted smartly to side-foot a cool shot into the gaping goal.
Rotherham manager Mark Robins made a switch at half-time to bring on big frontman Drewe Broughton in place of Ryan Taylor.
But it was the visitors who struck again against the off-colour Millers.
In a quick break, Gary Jones swept the ball out wide for Buckley who showed a burst of pace before lashing a low shot past Warrington.
Thorpe was sent off for a two-footed tackle on Sharps after 69 minutes.
Rotherham claimed a goal when Rory McArdle headed in his own net and then Reid smashed in the leveller four minutes into injury time.