Hart made 348 appearances for City between 2006 and 2016 and won two Premier League titles, two League Cups and the FA Cup during his time at the club.
Speaking on BBC Radio 5 Live's Monday Night Club, the 39-year-old said he had no reason to doubt Al Mubarak's account of the ongoing legal process.
"I've heard everyone else go down every single rabbit hole, every single wormhole, every single discussion," Hart said.
"My honest opinion is I've got no reason not to trust Khaldoon. What he said is enough for me."
Hart also praised the tone of Al Mubarak's letter to supporters, saying: "I loved how he spoke. I loved the letter because that was from someone who cared about the club."
Manchester City were accused of 115 breaches of Premier League financial regulations, with the allegations including claims that the club inflated sponsorship revenue from connected companies and made payments off the books to circumvent financial rules.
The club has now been found guilty of the majority of those charges and is expected to appeal. Sanctions have yet to be announced.
Hart said he did not believe the developments had tarnished his own achievements at City and stressed that the legal process was not yet complete.
"If Man City are found guilty, which I don't think they are because of what Khaldoon said, then there's conversations to be had," he said.
"But right now I'm very confident and very, very calm. I'm very proud of what I've achieved and what I did with Manchester City and what we achieved as a team. And that doesn't change. That doesn't change until you tell me."
Hart said he had not considered discussing the case with former City teammates because he did not want to become involved in speculation.
"I did what I did. I lined up 11 v 11 and had it out on the pitch. I won some games, lost some games, drew some games, and I shook hands and I walked off," he said.
"I know what I did, I know what we did, and I know how it happened. It didn't feel like we were scheming or plotting."