They took the lead in the 19th minute of Saturday’s League Two encounter at the Darlington Arena when teenage striker Curtis Main laid the ball into the path of Tadgh Purcell who fired low and hard into the far corner of the net for his third goal in four games.
Rotherham should have equalised five minutes later when Darlington keeper Shane Redmond raced out of goal and kicked his clearance straight to Danny Harrison, but with the goal unguarded he fired wide from distance.
In the 52nd minute, Darlington increased their lead with a first Football League goal for former Spennymoor midfielder Gareth Waite.
Main was involved again as he set up Mor Diop and a pass to the far post found Waite, who tapped in from close range.
In the 56th minute Darlington captain Ian Miller blocked an Adam Le Fondre shot as Rotherham looked to get back into the game.