Recalled striker Joe Mason bagged the only goal of the game - his fifth of the season - in the 41st minute, following up to tap home from close range after Jack Butland's parried save from Craig Conway had taken a fortuitous bounce.
It proved the only difference in what was a closely-fought encounter as Birmingham's threadbare side gave a solid account of themselves but ultimately failed to create clear-cut openings, with Marlon King, Nikola Zigic and Peter Lovenkrands all missing.
Cardiff, now with nine wins in their last 11 games, entered the year holding a five-point advantage at the summit and knowing the team leading the pile at the same stage in each of the last seven seasons have been promoted.
Mason's strike secured the Bluebirds' first win at St Andrew's since 1966 and their first league double over Birmingham in 46 years.
Despite their creditable performance it is a worrying time for Blues, who now boast just two wins in their last 11 and are four points above the drop zone entering a transfer window which could see some big names depart to ease their financial struggles.