Paul Mitchell, 32, was imprisoned in 2019 after knocking the England star to the ground when Aston Villa defeated their rivals at St Andrew's.
Mitchell served four weeks of his sentence after pleading guilty to assault and was also handed a 10-year ban from attending football matches as part of his punishment. He was sent back to prison in 2022 for breaching the football ban by watching Blues take on West Bromwich Albion.
The father of three was believed to have been found dead at an address in Birmingham earlier this week.
Social media was flooded with tributes for Mitchell.
Fellow Blues fan and friend Ash Herbert wrote on Facebook: "Absolutely gutted mate. We had some mad childhood memories I will cherish for the rest of my life, rest easy one of my best old friends mate. Till we meet again KRO."
Another fan posted: "Rip Paul Mitchell, football is everything, sad to see a fellow football fan taken so soon."
The attack on Jack Grealish, who scored the winning goal in the game after being punch, was greeted by universal derision by those within the game.
Grealish said after the match: “I was just unaware at the time. I think we had a corner or a throw-in and I was walking into position and I just felt a whack around the side of the face."