Neither side had played since December 12 and it showed as the first half was a drab affair, shaded in the early stages by Darlington, but neither goalkeeper was seriously challenged.
Darlington were restricted to breakaways and mistakes by Ian Sharps and Marc Joseph let in Tadgh Purcell and Jeff Smith but Andy Warrington saved with ease.
But two minutes after the break, Darlington grabbed a shock lead. Again Rotherham gave the ball away and Purcell marked his debut by cutting in from the left and drilling a low shot past Warrington for his first goal in League football.
And Darlington doubled their lead after 63 minutes when Rotherham’s defence was again caught napping by Mor Diop and his through-ball was forced home by Gary Smith.
Rotherham were given some hope after 78 minutes when substitute Micky Cummins fired home from close range after Law’s corner was only partially cleared, but Darlington held out and inflicted Rotherham’s first home defeat of the season.