Rooney's Birmingham - watched by minority owner Tom Brady - started positively with Jay Stansfield forcing Viktor Johansson into a low save inside three minutes.
Blues dominated the early exchanges, penning Rotherham back inside their own half.
However, a fingertip save by John Ruddy on 20 minutes kept the hosts from going behind as Sam Nombe battled past Dion Sanderson and got a low strike away which the goalkeeper did well to react to.
The visitors were gaining confidence and Christ Tiehi surged down the right wing and his low ball looking for the near post run of Nombe caused problems for the home defence and Emanuel Aiwu had to clear inside the six-yard box.
Nombe hit the crossbar for the Millers when the striker beat Ruddy to a high cross and saw his header crash against the woodwork.
Birmingham wasted a couple of excellent opportunities on the counter-attack, with the final piece of quality letting the home side down far too often.
Inside minutes of the restart Blues captain Sanderson made a crucial block, some suggested with his arm, on the line to deny Oliver Rathbone, after the Millers had disposed the hosts inside their own penalty area.
In front of a restless home crowd, Birmingham struggled to gain any momentum after the interval, and it was Rotherham the more likely of the two sides to make the crucial breakthrough.
Lukas Jutkiewicz was brought off the bench to try to add firepower to the Blues' frontline, but Johansson remained untested in the visitors' goal.
The experienced duo of Jordan Hugill and Sean Morrison came off the bench for Rotherham to see the game out as the hosts came to life and searched for a late winner.
Johansson had to beat clear a Cody Drameh effort but Ruddy had to make a crucial save at the other end to deny Nombe before Aiwu cleared to ensure the ball did not roll over the line.
Nombe nearly turned provider when his cross reached an unmarked Hakeem Odoffin but he could not connect with the ball and wasted a precious opportunity for the visitors.
Hugill then looped a header over the crossbar with 10 minutes remaining before the hosts thought they had won it through Jutkiewicz only for the offside flag to be raised.