Grealish was attacked while playing for former club Aston Villa against Birmingham at St Andrew's Stadium in the Premier League on March 2019.
Mitchell jumped over the barriers and ran onto the pitch in the first half of the match. He managed to punch Grealish from behind before being wrestled to the ground by stewards and taken off the pitch.
The England midfielder went on to score the winner for Aston Villa in a 1-0 victory for the visitors against their local rivals.
Birmingham condemned the incident after the match, apologised to Grealish and confirmed Mitchell had been banned from St Andrew's for life.
Mitchell subsequently pleaded guilty to assault at Birmingham Magistrate's court and was sentenced to 14 weeks imprisonment and handed a 10-year ban from attending football matches. He was released after serving four weeks of his term.
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His cousin Lucy Mitchell has paid tribute after Paul was found dead at his home, aged just 32.
“Paul absolutely adored his three children," she told The Sun. "He always had a smile on his face no matter what. He had the kindest heart. He is sadly going to be missed by a lot of people.
"He was a very popular man. It won't be the same without him here anymore."