Warnock, who spent eight years in charge of the Blades, had picked up two straight wins since moving from Crystal Palace two weeks ago.
And his Rangers side dominated for long spells by bouncing back after Richard Cresswell had scored against the run of play just before the break as Morocco international Adel Taarabt levelled matters in the 49th minute.
Taarabt, on loan from Spurs, had caused an early scare by lashing the ball across the face of the goal before he was stopped just inside the area when Mark Bunn dived bravely at the 20-year-old's feet as he was about to shoot.
United, whose home form has kept them in promotion scramble, should have snatched the lead after winger Mark Yeates picked out the unmarked Henri Camara but the former Wigan man side-footed over from 15 yards.
The Sheffield side escaped on 19 minutes as they failed to close down Hogan Ephraim, whose low strike from just inside the area had Bunn beaten before striking the foot of the post.
The elegant Taarabt carved out another opening as United creaked after robbing full-back Nyron Nosworthy before threading the ball to Ephraim, whose low drive was saved by the diving Bunn.
United, outplayed by the London side, broke the deadlock on 43 minutes.
Yeates cleverly found space before picking out Cresswell, who finished clinically by tucking the ball into the top corner of the net for his eighth of the season.
Rangers were back on level terms in the 49th minute when Taarabt shrugged off the weak challenge from Marcel Seip to slot the ball beyond the advancing Bunn, who minutes later had to tip a dipping free kick from Taarabt over the bar.
Camara caused a late scare in the Rangers defence after running across the face of the goal and finishing with a stinging shot but, with Carl Ikeme beaten, the ball flew wide of the target.