Donahue died surrounded by his family on Sunday following an illness, the NBC's Today show reported on Monday, citing a family statement.
Debuting in 1970 when daytime TV offered its mostly female viewers a diet of soap operas, game shows and homemaking programmes, Donahue's show tackled subject matter once considered taboo for television - including abortion, the sexual revolution and race relations.
With his boyish charm, irrepressible energy and thick white hair, Donahue was known for aggressively questioning his guests and bounding through the studio to give his audience a chance to be heard.
The success of his show paved the way for other daytime talk-show hosts, most notably Oprah Winfrey, whose programme me eventually eclipsed Donahue's in the ratings.
"If it weren't for Phil Donahue, there never would have been an Oprah show," Winfrey has said.
Among the proliferation of daytime shows following in Donahue's wake were a number that became known for sensationalism and occasional violence.
Such programme mes, hosted by personalities including Jerry Springer, Geraldo Rivera, Sally Jessy Raphael and Maury Povich were his "illegitimate children," Donahue told interviewers, adding he loved them all.
With the daytime talk field becoming increasingly crowded, loud and rude, Donahue's programme slid in popularity, leading to its cancellation in 1996 after 26 years and thousands of shows on national television, the longest run for a syndicated US talk show.
HOUSEWIVES' FORUM
At its height, Donahue's show was acclaimed by People magazine in 1979 as "a national forum for America's housewives."
"I think they appreciate the issues the show raises and enjoy the challenge of getting emotionally and intellectually involved in what's happening," Donahue told it that year.
"There are no prizes and nobody screams, we put on an honest sharing of ideas," he said of his show, which generally tackled one topic per hour-long episode.
Donahue, who often spoke of his Roman Catholic upbringing, was one of the first TV personalities to forcefully address sexual abuse of children by clergy in the Catholic Church, bringing the topic to national attention.
He first dealt with the sex abuse scandal in a 1988 episode and revisited it in later seasons of his show, giving victims a chance to tell their stories.
His later projects included hosting a talk show from 2002 to 2003 on the cable network MSNBC and co-directing the 2006 documentary film Body of War that took a critical view of the US invasion of Iraq, focusing on an American soldier who was paralyzed in the war.
In addition to hot-topic issues, Donahue occasionally devoted time to lighter fare like misdiagnosed allergies and traded quips with celebrity guests from comedian Jerry Lewis to shock rocker Marilyn Manson. For an episode on cross-dressing, Donahue wore a skirt.
He won nine Daytime Emmys for best talk-show host.
Born on December 21, 1935, in Cleveland and raised in that Ohio city, Donahue was the son of a furniture salesman and a department store clerk.
After graduating from the University of Notre Dame, he worked his way up in broadcasting until he was given the chance in 1967 to host his own programme, The Phil Donahue Show, on a Dayton, Ohio, television station. He caused a stir with some viewers by inviting an atheist as his first guest.
The show gained national syndication in 1970. He moved the show to Chicago in 1974 and then to New York in 1984 to be closer to his second wife, actress Marlo Thomas, the daughter of actor-comedian Danny Thomas.
Donahue's first marriage to Marge Cooney ended in divorce in 1975. They had four sons and a daughter. He married Thomas in 1980.
