Leeds United suffered their first defeat of the season on Saturday.
Monk returned to Elland Road and left with a hard-fought three points, achieved by two first-half goals from Che Adams.
Ezgjan Alioski pulled one back on 85 minutes for Leeds but it was a mere consolation.
Prior to kickoff, no team had beaten Marcelo Bielsa's top-of-the-table United, and Monk, who spent a year as Leeds boss, said on Twitter that he was proud of his troops.
Monk has been a villainous figure among many Leeds fans since May 2017, when he suddenly resigned as manager, before taking the job at Middlesbrough in a move which increased the hostility from the fanbase.
Here's how some Whites supporters responded to their former manager on Twitter:
Yep well done ...Judas— GLENN POTTER (@THEGUVNAR) September 22, 2018
You won a very dull football match, calm down— Jamie McEvoy (@JamieMcEvoy) September 22, 2018
What a joke— Gareth Rushton (@rushtonlufc) September 22, 2018
Hope u drop, the most boring team ive ever seen— Tomas Lufc Sweden (@LodetTomas) September 22, 2018
No need for that. If your proud of time wasting like that your not right. I think we were really poor today although you did your job.— Mitchell Emmerson (@MitchellEmmers2) September 22, 2018
Bielsa's side remain top of the Championship in spite of the defeat, but they're only ahead of Boro on goal difference.
Birmingham are nine points back in 17th place.