Dougie Freedman’s men were nowhere near their best but claimed victory thanks to two goals in two first-half minutes to leave them eight points adrift of the top six with eight games to play.
Dexter Blackstock struck his third goal of the season with a lobbed finish in the 43rd minute before Michail Antonio doubled the lead with a fine solo effort and those goals proved more than enough.
Rotherham, who are six points above the drop zone and still in a relegation dogfight, more than matched Forest and could have made a game of it had Lee Frecklington’s effort early in the second half gone in instead of hitting the post.
Connor Sammon was also denied by Karl Darlow’s fine save as boss Steve Evans was left to rue that manic two-minute period just before the break.
It had started as an even contest, with the Millers enjoying the lion’s share of possession, though they were unable to create anything in the final third.
Antonio fired a couple of warning shots from distance but he did not test Adam Collin in the Millers net as Forest struggled to find their groove.
But, with nothing to separate the two teams, the game was decided in 113 seconds on the brink of the interval.
First Antonio claimed an assist as he sent a long ball over the top which Blackstock got to first and he lobbed a back-pedalling Collin, who was perhaps out of position and could not keep the ball out despite getting a hand on it.
With the Forest crowd barely back in their seats they were celebrating again moments later when Antonio found his range – he beat several challenges in a surging run before unleashing a low drive from 20 yards which arrowed into the bottom corner.
That two-goal salvo flattered Forest as the Millers had competed well throughout the opening 45 minutes but it left them with a mountain to climb after the break.
They put up a fight, though, as Frecklington twice went close, first hitting the post with a volley on the edge of the area and then seeing his goal-bound effort superbly blocked.
The Millers’ pressure meant they were vulnerable to the counter-attack and Antonio was a constant threat with his pace and power, prodding an effort wide while Jamaal Lascelles planted a header wide.
Rotherham should have pulled a goal back in the 74th minute but Forest keeper Darlow, called into action for the first time, produced a superb reaction save to deny Connor Sammon.
And, when Frecklington missed from the resulting corner, Evans’ men knew it was not going to be their night.