Former Chesterfield striker Doyle got his first in nine games since his #1million switch to the Bluebirds just after half-time, before a Peter Whittingham penalty wrapped up the points.
It ensured Cardiff’s first win at home since January 10 to ease the pressure on manager Russell Slade.
Birmingham boss Gary Rowett was sent to the stands for the visitors in the second half and they could face further trouble after their fans clashed with police inside the Cardiff City Stadium.
Cardiff’s afternoon had got off to a bad start before kick-off when in-form Federico Macheda, with three goals in his last three games, pulled up injured in the warm-up and had to be replaced by Kenwyne Jones.
The home side started brightly enough and centre-back Sean Morrison went close with a header from Whittingham’s corner after three minutes but it proved to be a rare early opening.
Aron Gunnarsson went close to breaking the deadlock after 32 minutes when he met Joe Ralls’ cross from the right with a firm header but it flashed past the post.
Ralls almost got on the scoresheet himself 60 seconds later but his long-range effort was narrowly wide.
Demarai Gray was causing problems down the left for Birmingham but they failed to turn any of his good work into chances.
Cardiff’s troubles got even worse just before the interval when striker Alex Revell injured himself sprinting for a ball over the top and had to be replaced.
Birmingham then started the second period the better side but Cardiff’s breakthrough goal came from a failed attack by the visitors.
Clayton Donaldson was sent through on goal but when he hesitated Bluebirds defender Bruno Manga was able to dispossess and start a counter-attack.
Matthew Kennedy, on as a second-half replacement for Gunnarsson, found Doyle and he scored with the game’s first shot on target, in the 50th minute.
It sparked Cardiff’s best period of the game and they would have gone further in front but for the wayward shooting of former Stoke man Jones.
First he saw a shot saved by Birmingham goalkeeper Darren Randolph, before he wasted two good chances in front of goal, the best of which he put over the bar from six yards.
Doyle almost got a second when he combined with Kennedy and put the ball through Randolph’s legs, but his shot came back off the post.
The game was put beyond Birmingham when Doyle was brought down in the box by Jonathan Spector after 75 minutes and Whittingham stepped up to slot home the penalty.
Sky Tickets are giving you the chance to experience live Championship football. Buy tickets & get a free NOW TV Sky Sports Pass PLUS you could win Soccer AM tickets.