Goals from Tadgh Purcell and Gareth Waite earned three points for Steve Staunton's men as Ronnie Moore's visitors were made to pay for a poor display.
Darlington took the lead in the 19th minute when teenage striker Curtis Main laid the ball into the path of 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 Harrison fired wide from distance.
Darlington increased their lead seven minutes after the break as former Spennymoor midfielder Waite grabbed his first league goal.
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.
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