He died at 5:30 a.m. in a Palm Beach Gardens hospital, said his wife, Genevieve Douglas. She wasn't sure of the cause, but said he had been admitted Thursday.
Douglas became dehydrated on the golf course a few weeks ago and had been treated on and off since. "He was coming along fine, we thought. It was really a shock," she said. "We never anticipated this to happen."
Douglas' afternoon show aired from 1961 to 1982. It featured his ballad and big-band singing style, other musicians, comedians, sports figures and political personalities, including seven former, sitting or future presidents.
"People still believe 'The Mike Douglas Show' was a talk show, and I never correct them, but I don't think so," Douglas said in his 1999 memoir, "I'll Be Right Back: Memories of TV's Greatest Talk Show."
"It was really a music show, with a whole lot of talk and laughter in between numbers."
Douglas did about 6,000 shows, most 90 minutes long, and estimated that at its peak, the syndicated show was seen in about 230 cities.
Tom Kelly, who co-authored Douglas' memoir, said he had about 30,000 guests appear on his show over the years.
"One big key to his great success was he had his ego in check," Kelly said. "He always let the guest have the limelight. He was a fine performer. He could sing, he could do comedy; he did it all, but he always gave the guest the spotlight."
Douglas was among the "early settlers" in daytime talk shows, said Robert Thompson, a professor and director of the Center for the Study of Popular Television at Syracuse University's S.I. Newhouse School of Public Communications.
"Mike Douglas was an old-fashioned traditionalist, holding down the fort while the culture was changing," Thompson said. "He was always the very friendly talk show host, nice to everybody. He would lean toward his guest as if he really cared. He owned that territory."
Hosts Phil Donahue, Dinah Shore and Merv Griffin also found success about the same time. Douglas said in his book that people often confused him with Griffin, another singer of Irish heritage.
Tim Brooks, television historian and executive vice president of research for Lifetime Television Network, said Douglas was "an outgrowth on the 1950s mentality of politeness."
"Even when America was getting kind of angry in the 1960s and 1970s, his show was sort of an oasis of politeness," Brooks said. "It got you away from some of the turmoil in life."