Seven-time Super Bowl champion Brady, who recently became a minority owner at the English second-tier football club, made his appearance at The Roost pub near St Andrew's before the game against Leeds.
"Tom Brady is here and he's perfect," tweeted Kelsea Ravenhill, 26, from Birmingham.
"The film crew, Tom Brady, his bodyguards all entered the pub for his Blues entrance and introduced himself to all our fans," Ravenhill told Britain's Press Association news agency.
"The place is rocking, completely changed the atmosphere, he is just a sports legend!
"It's made everyone's day, was a pleasure to meet him and I just hope we can get the three points for him later now."
Brady has partnered with the club's holding company Knighthead Capital Management LLC and becomes chairman of a new advisory board.
The club says former quarter-back Brady will "apply his extensive leadership experience and expertise across several components of the club, including working alongside the sports science department to advise on health, nutrition, wellness and recovery systems and programmes".
Brady tweeted earlier Saturday: "Any plans before kick-off guys? See you at St. Andrew's". He then made an appearance at the stadium, where he was photographed meeting mascots and signing shirts.
Brady is the latest high-profile American to become involved in English football.
Hollywood stars Ryan Reynolds and Rob McElhenney bought Wrexham in 2020 and celebrated as the team returned to the English Football League in April.
Three-time golf major winner Jordan Spieth is involved at Leeds after buying shares in 49ers Enterprises, the investment arm of NFL franchise the San Francisco 49ers.
Championship side Birmingham have twice won the League Cup but the last time they did so, in 2011, they were relegated from the Premier League and they have not returned to the top-flight since.
