Steven Cann was the Millers’ hero, saving two penalties before Michael Cummins scored the winning spot-kick to dump his former club out of the competition.
The only bright spot in a disappointing first half came in the 34th minute when Darlington skipper Steve Foster turned the ball home from close range. Billy Clarke’s free-kick from wide on the right found its way to Liam Hatch and his cross was turned in by Foster.
Rotherham failed to manage a single shot on target in the first half, and Simon Brown’s only worrying moment came on the half hour when Peter Holmes put Ryan Taylor through, but the keeper got down smartly to save before Taylor could pull the trigger.
It was a different story after the break as Rotherham suddenly sprang to life. Reuben Reid and Taylor tested Brown, while Jamie Green had the keeper scrambling, but the home side had to wait until the 74th minute for an equaliser.
Dale Tonge’s cross from the right eluded everyone to reach Danny Harrison on the far post and he slammed the ball into the roof of the net.