After picking up a credible point at third place Reading at the weekend, Ainsworth and his side went one better and beating Alex McLeish's men thanks to a stunning strike from Italian Samuel Di Carmine.
At one stage it looked to be going wrong for QPR when they had Mikele Leigertwood sent off on the stroke of half-time, but they came out stronger in the second half and moved up to seventh in the table.
Birmingham could have been two up inside the opening five minutes if Camron Jerome and Kevin Phillips had their shooting boots on.
Mehdi Nafti played in Jerome in round the back of the QPR defence, but keeper Radek Cerny was equal to it as he tipped the ball round the post.
A minute later Phillips had an even better chance to give McLeish's men the lead with a volley, but again the Czech goal keeper was equal to it.
Birmingham suffered an injury blow when Stuart Parnaby was stretchered off and was replaced by youngster Jared Wilson - a late call-up to the bench after Garry O'Connor picked up an injury.
Rangers thought they should have had a penalty when Emmanuel Ledesma header across goal appeared to hit Liam Ridgwell on the armm but referee Stuart Attwell was unmoved.
Blues keeper Maik Taylor was not called into action until five minutes before the break when he was forced to tip over a Martin Rowlands free-kick.
Two minutes later Taylor was forced into action again when Di Carmine sidefooted a Lee Cook cross and the Northern Ireland keeper had to use his legs to get the ball to safety.
Things then turned sour for Rangers on the stroke of half-time when they had Leigertwood sent off for a high challenge on Lee Carsley and ref Attwell had no hesitation in showing the midfielder a straight red card.
But ten minutes into the second half Rangers took the lead through a spectacular strike from Di Carmine.
Damiano Tommasi cleverly flicked the ball to his fellow Italian and he turned and released a thunderbolt from 25 yards which flew into the top corner, giving Taylor no chance.
Philips looked to respond straight away but his close-range effort was blocked on the line by Gavin Mahon.
As the snow fell in west London an animated Flavio Briatore persisted in shouting instructions down to Ainsworth on the bench from the warmth of the directors box.
Philips thought he had levelled the game in stoppage time, but his strike was ruled out for offside and QPR held on the claim a vital win.