Douglas died in a Palm Beach Gardens hospital, after being admitted Thursday, said publicist and friend Warren Cowan. The cause of death has not been determined.
Douglas' wife of 53 years, Genevieve, told the Associated Press that her husband had become dehydrated while on the golf course several weeks ago and had been treated off and on since.
"He was coming along fine, we thought. It was really a shock," she told AP on Friday. "We never anticipated this to happen."
"The Mike Douglas Show," which was launched as a local program in Cleveland in 1961 and moved to Philadelphia a few years later, ran until 1982.
The 90-minute program, which in 1967 became the first syndicated show to win an Emmy, was daytime destination TV for millions of Americans.
"Dishes go unwashed and shirts remain unironed when Mike Douglas comes on," TV Guide once reported, referring to Douglas' housewife-heavy audience.
Viewers tuned in to see celebrity guests — an array that included Bette Davis, Richard Pryor, Muhammad Ali and a 2-year-old Tiger Woods, whose golfing demonstration prompted fellow guest and golfer Bob Hope to quip, "I don't know what kind of drugs they've got this kid on, but I want some."
Although the tenor of the Douglas show was usually light, Douglas also tackled more serious issues with a mix of guests that included Malcolm X, Richard Nixon, the Rev. Martin Luther King Jr., George Wallace and the imperial wizard of the Ku Klux Klan.
"The Mike Douglas Show" boasted a unique feature: a weeklong celebrity co-host or co-hosts, who included Fred Astaire and Jim Nabors as well as John Lennon and Yoko Ono.
With its mix of singing, dancing, skits, stunts and cooking segments, "The Mike Douglas Show" was more than just a talk show.
At the helm was the always gracious, always affable Douglas, who once told Time magazine: "I don't smoke, I don't drink, I get home every night. I'm square."
In 1966, during his talk-show host heyday, Douglas, the father of three daughters, scored a top-10 hit with his sentimental song about fatherhood, "The Men in My Little Girl's Life."
He also did the singing for Prince Charming in Walt Disney's 1950 animated classic "Cinderella."
Douglas, who always sang on his show — solo and with guests such as Sammy Davis Jr. and Sarah Vaughan — estimated that he was host for more than 6,000 programs and interviewed more than 40,000 people.
"I tried to make everybody look good and be comfortable because the better they looked, the better the show was going to come off," he once said.
Comedian Dom DeLuise, a frequent guest who twice appeared as a co-host, said Douglas "was skilled at getting the best out of you."
"He was a singer, he was an entertainer and he was funny, but he didn't mind acquiescing to you," DeLuise told The Times on Friday. "They'd create something so you'd look good."
On one show, DeLuise recalled, "they had me blindfolded. They had somebody's back there and I felt it, and I said, 'It's Mel Brooks.' When I took my blindfold off, it was an orangutan. It was 1 inch away from my lips, and he duplicated every facial expression I did; it was hysterical."
Burt Reynolds, another frequent guest, recalled that Douglas "just had a way about him that very, very few people have — that is, he knew how to listen."
"I hardly ever prepared anything," Reynolds told The Times on Friday. "We just winged it because he was so much fun and easy. It wasn't like he was racing you to the joke, which a lot of them do today. And he had a wonderful singing voice.
"He was very sadly missed when the show stopped."
Added Reynolds, who became a friend of Douglas: "He was so loved by everybody. I never heard anybody say anything negative about Mike."
The son of a railway freight agent, Douglas was born Michael Delaney Dowd Jr. on Aug. 11, 1925, in Chicago. At age 9, he began earning extra money by strolling into taverns and singing for the patrons.
"I just started singing Irish songs; you know they never miss," he told Irish America magazine in 2000. "And the coins just started flying at me. Even some paper money."
By 11, he was singing on a Chicago radio station, including an amateur program called "The Irish Hour." After graduating from high school, he worked as a singing master of ceremonies aboard a cruise ship on the Great Lakes.
He was attending Oklahoma City University during the day and singing at night on a local radio show before serving in the Navy during World War II.
After the war, Douglas became a featured singer with the Kay Kyser band, which included appearances on "Kay Kyser's Kollege of Musical Knowledge" on radio and television and the recording of songs such as "The Old Lamplighter" and "Ole Buttermilk Sky."
It was Kyser who changed Douglas' surname.
"He didn't even tell me," Douglas recalled in the 2000 interview with Irish America. "I was about to do a number, and Kyser introduced me as Mike Douglas. I had to look behind me to see if there was someone else about to go on."
In 1953, Douglas was back in Chicago, where he had a radio show called "Hi, Ladies." He also appeared on a TV variety show called "Club 60" and was a frequent guest on the network radio show "Breakfast Club."
By 1961 he was earning $125 a week singing in a Los Angeles nightclub and taking a course on how to succeed in real estate before he landed the job as a TV talk show host in Cleveland.
"This was definitely the last fling," he told Newsweek in 1967. "If it hadn't gone over, I don't know what I would have done."
When Rosie O'Donnell launched her own TV talk show in 1996, she told reporters at a news conference that Douglas was her talk show host "idol."
"When I was a kid, school just marked time between 'Mike Douglas Shows,' " O'Donnell wrote in the introduction to Douglas' 1999 memoir, "I'll Be Right Back: Memories of TV's Greatest Talk Show."
Among his memories: Jack Benny repeatedly turned down requests to appear on his show, saying he didn't want to do live TV. But the legendary comedian changed his mind after Douglas came up with an idea that Benny couldn't resist.
The day Benny was to appear on the show, Douglas and his camera crew went outside, where a crowd gathered to join him watching for the arrival of Benny's limousine — then a bus pulled up and America's favorite penny pincher stepped out, counting his change.
In addition to his wife, Douglas is survived by his daughters, Michelle Dowd, Christine Voinovich and Kelly Donohue; five grandchildren; and 10 great-grandchildren.
A private service for family members will be held Monday in North Palm Beach, Fla.
Instead of flowers, the family suggests donations be made to the Tiger Woods Foundation.