Referee Richard Clark was the subject of the 67-year-old’s ire, after sending off Joe Mattock deep into stoppage time - the second home player to go after Richard Wood had been dismissed 11 minutes from time.
Both players went for two bookings, Mattock’s second offence being - in Clark’s eyes - timewasting when taking a throw-in.
It ensured a bad-tempered end to a contest that never really got going, despite both sides having several chances in the first half.
Results elsewhere mean United are five points adrift despite battlong for a point, victories elsewhere for MK Dons and Bristol City meaning Warnock has all on to keep the south Yorkshire club up.
The game was a slow burner until the hosts suddenly sparked into life on the half-hour mark with three chances inside two minutes.
First, Chris Burke won a corner that Joe Newell swung over for Kirk Broadfoot to meet unchallenged at the back post. As the ball flashed across goal, Paul Green tried in vain to apply the final touch.
Sixty seconds later, Matt Derbyshire was played clear before he brought a smart save from Tomasz Kuszczak.
A huge appeal for handball from the home fans followed, after Danny Ward’s shot had been blocked by Michael Morrison but the officials were having none of it.
Birmingham offered little in the way f attacking threat during the first half, Jacques Maghoma’s cross that was spilt at the front post by Lee Camp being their best moment.
Three minutes after the restart, however, the Blues should have been ahead when Stephen Gleeson’s free-kick found Paul Robinson but the veteran defender headed wide.
Rotherham went close on the hour when Chris Burke was released down the right to cross for Derbyshire, who was blocked at the front post by Ryan Shotton.
The ball then ran to Danny ward but his first time shot was straight at Kuszczak.
Play raced down the other end and Camp had to save smartly from Clayton Donaldson as the game started to open up.
As if to underline this, the Millers were next to threaten as Joe Newell picked out Danny Ward but his header flew just wide.
Maghoma then had a great chance to break the deadlock when played through by a slide-rule pass from Jon Toral but Camp saved brilliantly.
Moments later, the hosts were down to ten men after Richard Wood collected his second booking of the afternoon for clattering Donaldson.
That left the Millers with 11 minutes to hold out and they did so, even when Mattock was also dismissed deep into stoppage time for time-wasting at a throw-in.
New Rotherham boss Warnock was impressed with what he saw, though, having only had one training session with his new side.
“I thought they did ever so well. I said to the lads if you think that’s exciting we’ve got 15 more of them,” he said.
“I thought they were excellent and on another day, we could have took the points but it wasn’t to be.
“In the situation we are in we just have to make sure we get 100 per cent out of the players and that’s what they did today.
“The lads fought to the end, I have nothing but praise for them. They can only do what they did, we need more points.
“We have to win games we are not expected to. It’ll be good to see what these lads can do.”