After giving them an early lead he pushed the game beyond Rotherham with an angled header in the final 15 minutes.
The referee twice had a look at the pitch before deciding the match could go ahead. It proved very much the right decision as both sides rapidly settled into a free-flowing game with none of the anxieties usually associated with sides struggling at the foot of their division.
City always looked to use their flank runners David Cotterill and Scott Murray, while United build their attacks around a fluent midfield.
First blood came to City after just 10 minutes. Cotterill made a good run before sending the ball to the towering figure of Bas Savage. He deftly turned it square to Brooker who found the bottom left-hand corner of the net for his seventh goal of the season.
Picking up the ball was a first touch for keeper Gary Montgomery, who had come in for Neil Cutler after he broke his jaw on Boxing Day.
Montgomery saw plenty of action and, in the space of a few seconds, twice denied Brooker another goal after a fast incisive run into the box by Murray.
Brooker, on top of his game, was a constant threat to Rotherham's security although their captain Colin Murdock had an outstanding game in the back four.
Rotherham went close to an equaliser when they were given a free-kick just inside the right touch line. Paul McLaren aimed for the near post where Martin Butler's header beat everyone but bounced off the far post.
They drew level when City's defenders were in all sorts of trouble after Matt Hayward missed his header from a McLaren corner.
Two players fell backwards into the net but couldn't keep out Murdock's strong downward header.
Murray put City back in front at the start of the second half racing into position as Brooker held up the ball and then Brooker rounded off a fine individual performance with his second of the night.