Unhappy at Real Madrid, Mourinho was keen to return to the Barclays 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.
Mourinho came to the consciousness of the British public when his Porto side won at Old Trafford en route to winning the 2004 Champions League, his charge down the touchline and knee slide following a last-minute goal becoming a memorable image of the modern European Cup.
That is his favourite moment at Old Trafford and he insists he never thought about being in the home dugout there.
"I never have these dreams," he said.
It was Mourinho who ended Ferguson's bid for a third European Cup, when his Real Madrid team won in a last-16 tie last term which saw Wayne Rooney left out of the line-up at Old Trafford.
Rooney is now a target for Chelsea, with a third bid to be made following Monday's match.
Mourinho, who is optimistic of completing a deal for Tottenham target Willian, added: "We are going to get a maximum of two players in this transfer window.
"If we get the second piece (Rooney), it will be the final piece."
Mourinho dismissed the theory his interest in Rooney was designed to unsettle United.
After signing goalkeeper Mark Schwarzer, midfielder Marco van Ginkel and forward Andre Schurrle, with another forward in Willian close to joining, Mourinho says improving his options at striker was a priority.
United have insisted time and again that Rooney is not for sale.
Asked why Chelsea will bid for a third time, Mourinho said: "We make the bid and they say no again. What do we lose? One email."
It has been suggested that Juan Mata could be a make weight in the deal.
Mourinho on Friday insisted the Spain playmaker was not for sale, but what if United want Mata?
"That's hypothetical, because they didn't ask," Mourinho said.
"The two answers we got from Man United is 'we don't accept the bid', 'we're not interested in selling the player'.
"That's the only official contact we had with Man United.
"Till Monday, we have nothing more to add to it."
It appears United are even less willing to do business with a domestic rival.
Mourinho used the example of Filippo Inzaghi, who played for Juventus and Milan, to emphasise such a scenario does not happen in Italy.
"Sometimes you push players to play abroad when you should be interested in keeping the players in your league," Mourinho said.
"When you keep players in your league, you are contributing to making your league the best league."
Rooney is reportedly unhappy at United.
Asked what he would do with an unhappy player, Mourinho said: "There is not a rule to a player that is unhappy.
"Many times they are unhappy because they want new contracts.
"Other times they are unhappy because they want to leave. And that's a different situation.
"Sometimes they are happy and the agents are unhappy. It's all about a new contract, more money. And after two days they are happy again.
"Other times it's because they really want to go."