There were less than nine minutes on the clock when the Villans midfielder found himself at the centre of shocking scenes.
Having chased down a ball from a throw-in close to the corner flag, Grealish walked away with his back to the Blues supporters in that part of the stadium.
A pitch invader then entered the field of play and proceeded to strike the 23-year-old from behind.
Grealish was caught off guard – as were the other players in the area and security officials.
After being left on the turf, stewards were able to get to the individual in question.
Villa and Birmingham players also confronted the man involved as he was dragged back towards the stands and into the arms of local police.
West Midlands Police later tweeted to confirm that a man had been arrested and was en route to a custody block "following the incident on the pitch involving an attack on an Aston Villa player".
😡 - "An absolute disgrace!"— Sky Sports Football (@SkyFootball) March 10, 2019
A fan has run onto the field and attacked Jack Grealish from behind. pic.twitter.com/PI1Wf3qXfP
To Grealish’s credit, he quickly shrugged off the incident.
As chaos erupted around him, the Villa skipper merely adjusted his shin pads and got back to his feet.
He was then involved in a brief exchange with the match officials before play got back underway.
Few could have blamed Grealish had the incident thrown his concentration and left him asking questions at St Andrew’s. He was, however, eager to get the game going again within a matter of seconds.
Grealish is a key man for Villa in their ongoing push for a play-off spot in the Championship.
He has missed plenty of football this season through injury, but has been a talismanic presence in recent outings after stepping back into the fold.
Dean Smith’s side are fully aware of his value, having seen him spark talk of interest from the likes of Tottenham over the summer.
Birmingham, and their supporters, also knew that Grealish could be the man to dash their dreams on derby day.
A prominent figure for Villa has been involved in several meetings with the Blues in the past, but Sunday saw him take in a first at the home of arch-rivals.
It is not an outing that he is likely to forget in a hurry.
While initially it appeared as though that would be for all of the wrong reasons, Grealish went on to make the most telling contribution of the game.
As fate would have it, he netted what proved to the match-winner in the 67th minute as he crashed home a left-footed effort which sparked wild celebrations and saw him join more supporters - Villa's this time - in the stands.
Birmingham issued an apology to Grealish after the game, with a statement on their official website reading: "Birmingham City Football Club would like to apologise to Jack Grealish and Aston Villa Football Club for an incident in this afternoon’s derby match.
"We deplore the behaviour of the individual who committed this act and rest assured he will be banned from St. Andrew’s for life. The Club will also support any further punishment this individual may face in the eyes of the law.
"The Club will be working with the relevant authorities to investigate all the circumstances and we will be reviewing our stadium safety procedures.
"What happened has no place in football or society. Jack is a Birmingham lad and regardless of club allegiance should not have been subjected to this – there are no excuses.
"Again, we apologise to Jack and all at Aston Villa Football Club."