Cooper died Wednesday morning in her sleep, her son the actor Corbin Bernsen wrote on Facebook. The family confirmed the death to CBS, according to a network spokeswoman.
She was in a Los Angeles-area hospital, according to Bernsen's spokesman, Charles Sherman, who said the cause of death was not immediately available.
"One of the last great broads in our business -- Jeanne Cooper, Mom -- is now stirring up trouble in great beyond," her family said in a statement.
Cooper will be remembered "as a daytime television legend and as a friend who will truly be missed by all of us here at the network," said Nina Tassler, president of CBS Entertainment, adding that the actress brought "indelible charm, class and talent to every episode."
"Heaven just gained one feisty angel," cast member Melissa Claire Egan posted on her Twitter account.
"A very sad day for all of us. You will be deeply missed," tweeted Jessica Collins, also on the serial.
Cooper joined the daytime serial six months after its March 1973 debut, staking claim to the title of longest-tenured cast member. The role earned her 11 Daytime Emmy nominations and a trophy for best actress in a drama series in 2008.
"God knows it's claimed a big part of my life," she told The Associated Press in March as CBS' "The Young and the Restless" celebrated its milestone 40th anniversary.
As the years passed, Cooper brushed aside thoughts of saying goodbye to the show and its fictional Wisconsin town of Genoa City.
"What would I do? I'm no good at crocheting. My fingers would bleed," she told the AP last September, with her 84th birthday approaching the next month.
But on April 12 Bernsen tweeted that his mother faced an "uphill battle" for an undisclosed illness. In subsequent days he wrote of her gradual improvement and said that she'd been taken off breathing equipment.
In a Facebook posting April 17, Bernsen said his mother cursed several times, "showing me that she's becoming her old self, not thrilled about the situation, and ready to get out of the hospital and shake up the world."
On Wednesday he wrote that she remained a fighter until the end: "She has been a blaze her entire life, that beacon, that boxer I spoke of earlier. She went the full twelve rounds and by unanimous decision... won!"
Cooper, born in the California town of Taft in 1928, attended the College of the Pacific and performed in local theater productions before her professional career began with the 1953 film "The Redhead from Wyoming" starring Maureen O'Hara. Other film credits include 1968's "The Boston Strangler" with Tony Curtis and 1967's "Tony Rome" with Frank Sinatra.
She had a parallel career in TV, with shows including "The Adventures of Kit Carson" in 1953 and "The Pepsi-Cola Playhouse" in 1954 and "Bracken's World" in 1969-70.
In a recurring role on "L.A. Law," she played the mother to Bernsen's character, Arnie, and received a 1987 Emmy nomination for best guest actress in a drama. Bernsen later joined his mother on her series, making several appearances as a priest, Father Todd.
But it was her role on "The Young and the Restless" that made her a TV star intimately familiar to viewers.
In 1984, Cooper's real-life facelift was televised on the show as her character underwent the surgery at the same time, and had no regrets about it.
"It opened up reconstructive surgery for so many people, youngsters getting things done," she said. "To this day, people will come up to me and say, `Thank you so much for doing that. My mom or I had something done, and not just cosmetic surgery.' That was an incredible experience in my life."
"The Young and the Restless" has topped the daytime serial ratings for more than 24 years, in part because of the continuity provided by Cooper and its other long-time stars including Eric Braeden. It held its ground as the genre diminished in popularity and the majority of soaps vanished.
Cooper's 30-year marriage to Harry Bernsen ended in divorce. The couple have three children, Corbin, Caren and Collin, and eight grandchildren.
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