Martin Butler's strike just before the hour was enough to settle a game mostly played in driving rain.
Chesterfield huffed and puffed but rarely troubled the visitors who claimed three well-deserved points to send their hundreds of soaking supporters home happy.
The Spireites' best spell came in the opening 25 minutes when Paul Hall fired into the side netting and Tcham N'Toya-Zoa was bravely denied by Neil Cutler when through on goal.
But Rotherham slowly took control and Lee Williamson's side-footed effort against the bar on 39 minutes was a sign of what was to come.
Hartford's men continued their dominance following the restart and made a decisive breakthrough with the game's only real moment of quality.
A wonderful passing move led to Williamson breaking free down the right and his superb low cross was turned in at the far post by the experienced Butler for his second goal of the campaign.
Butler and Deon Burton both came close to extending United's lead before substitute Colin Larkin missed a sitter to level for the home side on 83 minutes.
Williamson went down under a heavy challenge from Derek Niven and with the Millers defenders expecting referee Graham Lows to whistle, Larkin broke free but fired badly wide.
It would have been harsh on the Yorkshire side but they played out the remaining minutes comfortably to kick-start their season at the third attempt.