The 23-year-old scored in the first half as Dean Smith’s men recorded a memorable victory.
The midfielder met Albert Adomah’s cross at the far post to head Aston Villa in front before half-time.
Grealish was reduced to tears amid passionate celebrations.
He dedicated the goal to his baby brother Keelan Grealish who died when the star was just four.
“I had tears in my eyes,” he said.
“I was crying because all my family were here and it’s a moment I dreamed of as a fan.
“I got emotional after scoring because of my little brother who passed away. I dedicated the goal to him.
“Ever since I was a little kid at school, these were the games I dreamed of playing in, but to score brings it to another level and it is probably the favourite moment of my career so far.”
Keelan lost his life back in April 2000 unexpectedly due to cot death.
Now Grealish uses his brother as inspiration every time he steps across the white line.
“I don’t think I’ve ever scored with a header in my life, but Albert Adomah put a great ball in at the back stick and I couldn’t really miss,” he added.
Villa’s win was crowned with a sweet moment when Alan Hutton went on a marauding run before slotting home.
“It was unbelievable, wasn’t it? I was right next to him and when he ran past Craig Gardner I thought Craig was going to give him a kick,” Grealish continued.
“He just carried on – I know how much it means to Hutts, he’s a great player and he’s been a great servsant to this club.”
Villa are now up to 8th place and four points behind Derby who occupy the final play-off position.
Birmingham drop to 12th place on a frustrating day for Garry Monk’s side.