Moore said she had known of the disease for about two to three years, as did many other of Dame Barbara's friends, including David Walliams and Christopher Biggins.
Her husband, Scott, 55, has now detailed her condition which he has revealed was diagnosed in 2014.
Mr Mitchell said he first noticed something may not be right in 2009 - just before his wife quit Eastenders - where she played Peggy Mitchell - the first time.
She took on the matriarch in 1991 when she claimed the Queen Vic pub in Albert Square.
He stressed that revealing the news any earlier would have been detrimental to her health.
The couple agreed that it was a good idea to kill off her character.
However, Scott went to speak to Dominic, revealing: "So I went to see him and, without giving the full situation, confided that she was really struggling to learn lines and wouldn't ever be coming back again after this".
She explained: "You can be relieved to know that's what it is and then you can begin to plan - but it is a terminal illness and it is sad". But this is the worst possible thing you can do to someone with Alzheimer's, as I learned from reading psychologist Oliver James' brilliant book, Contented Dementia.
The Pat Butcher actress continued: "My thoughts have been with both them all the time and I'm sorry to say I haven't seen Barbara for a few months".
"I hope speaking out will help other families dealing with loved ones who have this cruel disease. I'm doing this because I want us to be able to go out and, if something isn't quite right, it will be okay because people will now know that she has Alzheimer's and will accept it for what it is".
Jane added that Scott made the decision to make the announcement because he and Barbara still wanted to go out in public.
"Since her 80th birthday last August, a definite continual confusion has set in, so it's becoming a lot more hard for us to hide", he added in the interview".
"Many of us know Alzheimer's as a disease that causes memory loss like forgetting names or the day of the week", Aileen Waton, head of dementia for Bupa Care Services explains.
Dame Barbara, who is 80, is among 225,000 people who develop dementia each year in the UK.
For most people with Alzheimer's, the first symptoms emerge in the form of memory lapses.
But I soon learned she was still the same person.