Carl Asaba struck his 10th of the season as the FA Cup semi-finalists bounced back from their Old Trafford misery to stake a firm claim on a play-off spot.
Asaba, making his first start since January, hit the target in the 62nd minute, leaving Neil Warnock's high-flyers needing just four points from five games to guarantee a top-six finish.
Forest, also chasing the play offs, had the edge early in the game. Marlon Harewood sliced through the United defence on 11 minutes, but with David Johnson inside he chose to go alone before slipping as he tried to twist past Phil Jagielka.
United escaped again minutes later, Andy Reid whipped in a wicked centre with Harewood and Johnson failed to connect as the ball flashed across the goalmouth before the Bramall Lane side almost snatched the lead.
Midfielder Michael Tonge curled a free-kick round the wall as keeper Darren Ward flung himself to his left to fingertip the ball around the post.
Michael Brown ran half the length of the field evading two tackles before picking out Dean Windass as he steamed into the area but he hacked the ball over from 15 yards.
Windass shot wide and Peter Ndlovu's close-range header from Mark Rankine's cross flashed wide of the target as United took control.
The breakthrough almost came in the 34th minute, Jagielka outjumping his marker to stretch for Tonge's corner, his fierce header beating Ward but crashing against the underside of the bar before the ball was hacked clear.
Ndlovu should have broken the deadlock a minute later, Tonge piercing the Forest back four with a delicate pass as the United midfielder, with time to pick his spot, hit a tame shot from 12 yards.
Johnson produced a touch of magic minutes before the interval meeting Jim Brennan's cross and finishing with an overhead kick, pushed round the foot of the post by Paddy Kenny.
Asaba swept United ahead in the 62nd minute, the goal carved out by the hardworking Ndlovu, as he weaved into the box before threading the ball to the big striker who toe-poked it past Ward from 12 yards.
Jack Lester headed wide and Jon-Olav Hjelde's close-range effort was fisted away by Kenny as Forest battled in vain for an equaliser.