Alex McLeish's side were gifted the lead when Burnley goalkeeper Brian Jensen pushed Cameron Jerome's close-range effort into his own net before Ecuador striker Christian Benitez increased Birmingham's advantage in the final moments of the first half.
Steven Thompson got one back for Burnley late, but the loss compounded their pain after last weekend's confirmation of relegation just a year after winning promotion via the Championship playoffs. The Clarets' woeful away form has been key to their failure - and this was their 17th defeat in 19 away league games.
Burnley's fate was sealed by defeat against Liverpool and Brian Laws's side seemed to lack any real motivation.
Birmingham, ninth in the table, were unbeaten at St Andrew's since September and laid siege to the Burnley goal. Birmingham's pressure paid off as they took the lead in the 29th minute.
Any hopes of a Burnley fightback were ended four minutes before halftime as Benitez celebrated his 24th birthday in style with his first goal at St Andrew's.
Caldwell brought down Jerome to the right of the Burnley box and McFadden's freekick was headed back across goal by Roger Johnson before Benitez applied the finishing touch from close range.
Burnley began the second half on the offensive and Steven Fletcher fired over before bringing Joe Hart into action with two shots in quick succession. Their improved attacking was rewarded in the 87th minute when Thompson beat the offside trap and shot past Joe Hart.
In later games, Man City beat Aston Villa 3-1, Portsmouth beat Wolves 3-1, Spurs beat Bolton 1-0 and Stoke and Everton had a goalless draw.
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