The visitors come into this game having lost only once this season - a 2-0 away defeat to fellow high-flyers Cardiff.
Their star performer here was young Swedish left-back Sebastian Larsson, who linked up nicely down with August signing Gary McSheffrey.
And it was McSheffrey's telling set-piece which provided the opener on 23 minutes. He swung in a teasing free-kick which experienced defender Bruno N'Gotty headed in unmarked at the back post.
But QPR responded well and were soon troubling the Birmingham defence. On 31 minutes, Lee Cook hit a good-looking volley on target and then Marc Nygaard had a penalty appeal turned down.
Strike partner Dexter Blackstock was also felled just outside the box by Damien Johnson on 40 minutes, and Marc Bircham forced Maik Taylor into a good low save from the resulting free-kick.
If there was an area that needed looking at during half-time, it was QPR's suspect defence, which let Dudley Campbell through two minutes later. Luckily for them, his cool finish was ruled out for handball.
Although David Dunn also went close for Birmingham with a volley from a corner on the stroke of half-time, it was a pretty even opening period.
But Birmingham were fast out of the traps and controlled the second half. First Dunn forced a flying save from Paul Jones with another long-range volley, and then came a succession of chances for Campbell.
He first should have done better after latching onto a throughball from Dunn and then hit the post from point-blank range after a pull back from Nicklas Bendtner on 65 minutes.
But this seemed to frustrate the tall Dane more than his strike partner, and his following antics could have cost his team.
He picked up his first booking a minute later for needlessly arguing with the referee, and then slid in on Bircham two minutes later to seal his early bath.
Fortunately for him, it had little impact on the game and Birmingham continued to dominate, with McSheffrey going close twice in the closing stages.
Substitute Cameron Jerome confirmed the three points in stoppage-time with a clever lob over Jones – his first goal since his move to St Andrews from Cardiff in the summer.