It was a return to winning ways for Ajayi and his West Brom teammates, after playing a disappointing 1-1 away draw against Wigan Athletic on Thursday.
Birmingham opened the scoring in the 3rd minute through Lukas Jutkiewicz before Grady Diangana equalised for West Brom on 10 minutes.
In the 47th minute Harlee Dean put Birmingham in front again but West Brom levelled up on 73 minutes through Charlie Austin.
And with nine minutes remaining in the game, Austin grabbed his second goal and West Brom’s third, to seal all three points.
The win temporarily move West Brom Back to the top of the Championship table on 49 points.
They are three points ahead of second placed Leeds United who will host Cardiff City later on Saturday.