The 80-year-old EastEnders and Carry On star was first told about it four years ago, but made a decision to keep it quiet until now, as her symptoms had grown worse in recent weeks.
'Rather than me living in fear she might get confused or upset, they'll know that if her behaviour seems unusual, it's due to Alzheimer's and accept it for what it is, ' he added.
Mr Mitchell, 55, said he wants fans of the Carry On legend to know why her behaviour may be erratic in the future.
Dame Barbara also appeared in sitcoms including Dad's Army and One Foot In The Grave and the kids' series Worzel Gummidge before securing the role of pub landlady Peggy Mitchell in EastEnders in 1994. "I'm doing this interview - and I would like to make clear that I'm not being paid for it and it's the only one I'll be doing - because I know that rumours are circulating in showbusiness circles".
By 2016, the forgetfulness and confusion were worse, and it was agreed she would leave EastEnders for the final time.
Scott explained that he was given Barbara's blessing to reveal her illness to the world, but if she soon forgets she did he would "have to deal with it". And I understood because who would want to take that in?
After her diagnosis, Mr Mitchell was determined to keep their lives as normal as he could.
'Ironically, I think it's some of the best work she ever did.
He stressed that revealing the news any earlier would have been detrimental to her health.
By 2012 she had begun to repeat stories and sentences, which was when she was told she should visit her doctor.
The couple have kept the diagnosis a secret until now, but Mr Mitchell said it's becoming more hard to keep under wraps as her condition has deteriorated further since his wife's 80th birthday in August a year ago.
"I don't want it to come across that she's sitting there unable to communicate, because she's not".
Carry On star Barbara cried when she was given her diagnosis and is beginning to struggle despite help from her husband. "We're still going out for walks or dinner with friends and we still laugh together a lot". He insisted going out is good for her, so he doesn't want her to feel uncomfortable being around people.