The Royals still sit sixth in the Championship table but are now just two points ahead of Bournemouth in seventh, while Boro are eighth.
Goals from former Boro man Lukas Jutkiewitz and Harlee Dean, either side of a Yakou Meite equaliser, gave new Birmingham boss Lee Bowyer a dream start after replacing Aitor Karanka this week.
Elsewhere in the Championship, Barnsley have won nine of their last ten league games after a 3-1 win at Wycombe – despite a red card for midfielder Alex Mowatt.
The Tykes are three points ahead of Reading in fifth and are showing little signs of letting up under manager Valerien Ismael.
Boro will hope to keep pressure on the sides above them when they travel to Millwall this weekend, with Neil Warnock’s side looking to make it three wins in a row before the international break.