Unhappy at Real Madrid, Mourinho was keen to return to the Premier League after leaving Chelsea in September 2007 and told Ferguson so.
"In the same way he told me he was going to retire and I kept secret on that, I also told him I was coming in this direction," said Mourinho, who takes Chelsea to play David Moyes' United on Monday.
"Not definitely, because I couldn't say that. It was not done and I had a contract with Madrid and I had to be free of that. But at the same time he knew I wanted to come to Chelsea."
Asked why Ferguson confided in him, and vice versa, Mourinho revealed the pair shared many secrets and regularly asked each other's advice on players.
"He thought this big decision he could share with me, the same way I told him that I was on my way to Chelsea," Mourinho added.
"We kept that (secret). We trust each other."
Mourinho did not believe Ferguson was sounding him out as a possible successor and was not aware Moyes, then at Everton, was in line to take the job.
But he was not shocked that Ferguson recommended his fellow Scot.
"Everybody in this country knows the job he was doing at Everton, so for me not at all was it a surprise," Mourinho said.