The factory owner, who has been one of the soap's most popular characters for the past 30 years, will be told he has Alzheimer's in Friday's episode.
He will die next month from pneumonia caught after he gets lost in the rain.
With about 14 million viewers expected to follow his demise, the charity is bracing itself for a deluge of calls and a potential fundraising bonanza.
The advert, which will be shown regionally on ITV and on cable channels until May, depicts the effect Alzheimer's has on an individual's brain and on couples' lives.
The charity has recruited 50 extra volunteers to staff its helpline tomorrow and on Friday, when GMTV will be running a number of programmes on dementia.
Neil Hunt, chief executive at the society, will be among the studio guests.
Some episodes of Coronation Street will conclude with a helpline number that gives people contact details for the society.
"This is the first time that a person with Alzheimer's has been shown on prime-time TV," claimed Gayle Ward, a spokeswoman for the charity.
"This will bring it out of the shadows. If the story is handled responsibly, it could make a real difference to how people think about dementia."
Alison Sinclair, a senior publicist for Coronation Street, said the soap always contacted expert bodies for help on storylines. "The society has helped us every step of the way," she said. "We would never do a storyline like this and assume we knew everything."
Johnny Briggs, who plays Mike Baldwin, has written a piece for the society's website, and Rupert Hill, who plays Baldwin's grandson Jamie, is encouraging people to join the charity's fundraising trek to Iceland in July.