Lewis Grabban had drawn Forest level in the first half after Michael Smith put the hosts ahead at the New York Stadium, but Michael Ihiekwe's second-half header secured a deserved three points for the hard-working Millers.
In truth, Forest never truly looked like securing a first away win since November, or back-to-back wins for the first time since September as they were out-fought by a Rotherham side probably in even more need of the points.
Forest made two changes from the side which fought back to beat Swansea City in their last outing, striker Daryl Murphy's goalscoring cameo from the bench earning him a starting spot, in place of Ryan Yates, while Saidy Janko came in for Tendayi Darikwa as Forest switched to a 4-4-2 with a diamond midfield of captain Ben Watson behind Matty Cash and Jack Colback, with Joe Lolley behind a strike pairing of Murphy and top scorer Grabban.
The game got off to an electric start as Forest got caught in midfield inside two minutes and Ben Watson was forced to throw himself in front of a goalbound shot in front of the raucous, 2,616 Forest fans - but the ref ignored the home fans' loud appeal for a penalty.
A minute later at the other end, a Cash long throw caused confusion and fell to centre back Yohan Benalouane, whose backheel come off the inside of the post.
The game was being played at a frenetic pace and it was the home side who drew first blood as Smith found space in the area to fire past Costel Pantilimon after another high ball caused confusion in the box.
Will Vaulks's long throws from the right wing were repeatedly causing problems and, with Forest attempts to play Grabban in through the middle repeatedly thwarted by the offside flag, it looked as if long throws from Cash might be The Reds' best hope for a goal.
It certainly led to the next chance, after 24 minutes, when a Cash throw was flicked on only for Grabban to volley against the post under pressure.
From goalkeeper Marek Rodak's clearance, the ball fell to winger Jon Taylor, but he volleyed over when well placed to double The Millers' lead.
And he was left to rue the miss as three minutes later Forest were level. Grabban battled for Pantilimon's long clearance and it fell to Lolley whose flick put Grabban through on goal. He fluffed his first shot, but it came back off Rodak and he was able to take a touch and tap into the empty net.
It was Forest who created the first good chance of the second half, after 53 minutes, as Grabban's low shot from Cash's cross was superbly tipped behind by Rodak.
At the other end, Semi Atayi headed the impressive Vaulks's left wing cross wide, when it looked easier to score.
But Rotherham did take the lead five minutes later to the delight of the majority of the 11,012 crowd, when Ihiekwe headed into the top corner after a left-wing corner was headed back across goal by Smith.
Forest threw on Arvin Appiah and Joao Carvalho for the last 15 mnutes in place of full backs Janko and Osborn - with Colback and Cash dropping back - in a bid to force an equaliser, but it was Rotherham who looked more likely to score as they hit Forest on the break as they held on comfortably to a deserved three points.
Forest's thoughts now turn to another trip to South Yorkshire and a visit to a resurgent Sheffield Wednesday at Hillsborough on Tuesday.