Earlier on in the tenth minute of an eventful game, Grealish had been attacked from behind by a Blues fan who ran on to the pitch.
Paul Mitchell, 27, who appeared before Birmingham Magistrates Court, also admitted invading the St Andrews pitch during the match and will be sentenced later on Monday.
Grealish responded to the incident with the game-winning goal in the second half of Villa's 1-0 victory.
In all, four people were arrested on suspicion of assault or violent disorder during the fixture.
"A fan running on the pitch and doing something like that". It was unbelievable, in front of our fans and after what happened in the first half.
"It's a situation no player should ever be faced with", the EFL, the second-highest division in English soccer, said in a statement.
Paul Mitchell being restrained after attacking Aston Villa's Jack Grealish on the pitch (right) during the Sky Bet Championship match at St Andrew's Trillion Trophy Stadium, Birmingham.
West Midlands Police confirmed in a brief statement that the spectator had been arrested and taken into custody. "We will be working closely with the Metropolitan Police in their investigation".
The event was one of two significant events yesterday, with another pitch invader entering the field of play during Arsenal's Premier League fixture against Manchester United. This is entirely unacceptable and we strongly condemn both incidents.
It's important to note that Villa-Birmingham is a derby match, a rivalry that means a heck of a lot to both the players and fans.
The club said there were "no excuses" for his behaviour and it was reviewing its stadium safety procedures.
In Friday night's 1-1 draw, Rangers captain James Tavernier found himself face to face with a Hibernian fan who had jumped out of the notorious East Stand at Hibs' Easter Road home.
When asked about what sanctions could be implemented, Taylor said: "There's a combination you can consider - fines, ground closures, playing games behind closed doors and points deductions".