The 71-year-old revealed his decision in-house after his team had built up a lead in the Premier League and admits that it was tough to keep the news under wraps.
The Scot sealed his 13th league title with the Red Devils this year and promptly announced that he would be stepping down in May, with David Moyes confirmed as his successor.
"Sometimes I was sitting with my family and you forget you're actually going to retire and talk about next year," he told MUTV. "You're afraid you're going to blurt something else but I didn't tell my sons until the end of March - well, Jason knew earlier as he works a lot with me.
"I only told my brother the night before it was announced and he wasn't too pleased! I got a bit of invective off him.
"It was difficult but the club knew in March. We sat down and discussed it and, of course, David [Gill] is retiring too and it gave me a problem as I didn't want to think we'd be leaving the club in the lurch.
Ferguson says it was tough to keep the news under wraps, adding that Old Trafford is a very difficult place to keep things quiet.
"The rumor started out, didn't it," Ferguson said. "On Tuesday night, the rumor had gone out and it's hard to keep things intact at Old Trafford. It's a bit of a sieve at times and news seems to creep out from all different angles and sources.
"I've tried over the years to eliminate these leakages of news getting out. I always thought, over the last 10 years especially, a part of my job is to keep us out of the news, which is nigh on impossible.
"You have to accept we are news but there's a difference between good news and bad news. I always tried to keep bad news away from the club and that's difficult in the modern day."