The revelation comes in a new three-part Netflix documentary series, simply titled "Mourinho," which traces the Portuguese coach's managerial career. Now 63 and back at Real Madrid for a second spell in charge, Mourinho used the series to lift the lid on a chapter of his career he says "nobody knows" about.
"When I left Real Madrid, I signed to go to Manchester United after Sir Alex," Mourinho said.
Ferguson, who won 13 Premier League titles across 26 years at Old Trafford before stepping down in 2013, backed up the account in his own contribution to the documentary.
"He was offered the job, yes," Ferguson said. "I sat him down and explained the situation. As far as I was concerned, he was accepting. And then, within hours, it changed."
According to Ferguson, Mourinho called him in visible distress to explain he could not go through with the switch, given his prior commitment to Chelsea, the club where he had already won back-to-back Premier League titles in 2005 and 2006.
"One night, he phoned me and he was crying and he says, 'Alex, I can't take it. I've given my word to Chelsea and I'm not going to break my word,'" Ferguson recalled. "The reason he gave to me, I could understand, but I was disappointed."
Mourinho, for his part, described being "very flattered" to have been lined up as Ferguson's successor, acknowledging the scale of the opportunity he turned down.
"Because Manchester United has an incredible appeal," he said. "But one thing is love for football, another thing is love for a certain club. I think this is more powerful than love for football and fundamentally, after Real Madrid, I needed to feel that I'm loved.
"For me to lose the chance to be the coach to succeed Sir Alex Ferguson was dramatic, but I don't regret it because it was a decision that I made with my heart."
David Moyes was ultimately handed the United job following Ferguson's retirement, lasting less than a season before his sacking. Mourinho, meanwhile, went on to win a third Premier League title and the League Cup in his second spell at Chelsea, before finally getting the chance to manage United in May 2016, taking over from Louis van Gaal.
He spent two and a half years at Old Trafford, winning the League Cup and Europa League, before being sacked in December 2018 following a run of poor results.