The cause was cancer, said her husband, Donald Webster.
Douglas was best known for her work in television, stretching back to the late 1940s, when she appeared in live dramas. She went on to roles in episodes of hit series such as "Naked City," "The Waltons," "Ben Casey," "The Streets of San Francisco" (with son Michael) and "The Paper Chase," on which she had a recurring part as a law professor.
One of her last appearances was on a 2004 episode of "The West Wing" in which she had a long, biting monologue as the widow of an ex-president.
Diana and Kirk Douglas divorced in 1951, but were reunited on screen as a married couple in the 2003 film "It Runs in the Family," which also featured son Michael.
To prepare for scenes with her ex-husband, she thought about the good times in their marriage. "It was useful," Douglas said in an About.com interview. "Actors are always looking for intense memories to call on."
She found memories of the breakup to be of use, too.
At the time they were divorcing, she was making the film "House of Strangers," and fellow cast member Luther Adler told her the sadness she was going through was "actor's gold."
"You should really relish it," Adler told her. "You're going to need it when you get older and you're going to use it as an actor."
Eventually, she and Kirk Douglas rekindled their friendship. "I always enjoyed Kirk's sense of humor, even when I was angriest at him," she said.
"He could always make me laugh, somehow, which annoyed the hell out of me."
She was born Diana Dill on Jan. 22, 1923, in Bermuda. Her very proper British father, who was the island's attorney general, woke the family at 5:30 a.m. with a bugle.
From an early age, Douglas was rebellious. "I broke all the rules," she said in a 1999 New York Post interview. "If a sign said, 'Walk, don't run,' I definitely ran."
She was sent to boarding school in England and graduated from the Upper Chine School for Girls on the Isle of Wight. Though her father wanted her to become a lawyer, she enrolled in the American Academy of Dramatic Arts in New York. It was there she met Kirk Douglas at 16.
Before graduating, she signed a contract with Warner Brothers and moved to Los Angeles. In her frank 1999 memoir "In the Wings," Douglas said she had an affair with Errol Flynn before marrying Kirk Douglas in 1943.
A few years after they divorced, she appeared in "The Indian Fighter," the first film made by his company. It was not as happy an experience as on "It Runs in the Family," nearly 50 years later.
"It was a damn sight easier this time," she told Entertainment News in 2003. "We're both older and wiser now."
Diana Douglas' second husband, producer and writer William Darrid, died in 1992. She and Webster, who was a top Treasury Department official in the
In addition to her husband, she is survived by sons Michael and Joel; and three grandchildren.