Patty Duke, known for her Academy Award-winning performance in "The Miracle Worker," has died at 69. Here's a look back at her acting career and advocacy work. (Nicki DeMarco/The Washington Post)

Patty Duke, a onetime child actress who won an Oscar at 16 and overcame an exploited youth and ravaging mental illness to excel in television, theater and film for more than half a century, died March 29 at a hospital in Coeur d’Alene, Idaho. She was 69.

Her death was reported by the Associated Press. The cause was sepsis from a ruptured intestine, according to her agent, Mitchell Stubbs.

Ms. Duke began acting in grade school in TV shows and commercials and was propelled to fame when she received the Oscar for best supporting actress for her performance in “The Miracle Worker” (1962) as a young Helen Keller, the author and activist who was deaf and blind. Anne Bancroft played Keller’s untiring instructor, Annie Sullivan, and received the Oscar for best actress.

The two women had previously appeared together in a Broadway production of “The Miracle Worker” written by William Gibson, with Ms. Duke featured alongside the more prominent Bancroft on the marquee.

On stage and on screen, Ms. Duke stunned audiences with her ability to mimic the blank gaze of the blind. With poignant verisimilitude, she showed Keller breaking through the agonizing isolation of her disability, a triumph encapsulated in a memorable scene in which Sullivan, vigorously working a pump, watches as her pupil holds her hands under the spout and slowly utters the word “water.”

Anne Bancroft, left, and Ms. Duke perform in the Broadway play “The Miracle Worker” in 1959. (AP)

Ms. Duke continued her rise on television, starring from 1963 to 1966 in the ABC sitcom “The Patty Duke Show.” The young actress played two roles — Patty Lane, a bouncy American teen, and Cathy Lane, her identical, brainy Scottish cousin who comes to live with the Lanes in her Brooklyn Heights home.

Their adventures and misadventures bordered on the absurd but firmly established Ms. Duke — as well as the show’s perhaps too-catchy theme song — in the collective memory of the 1960s.

Unbeknown to viewers of the peppy show, Ms. Duke’s young celebrity had been engineered in large part by her managers, John and Ethel Ross. Years later, Ms. Duke would accuse them of giving her alcohol and prescription drugs and sexually molesting her.

The stage, she once told an interviewer, was her “safe haven” because “they couldn’t come up there and make me crazy.”

Proud of her accomplishments in “The Miracle Worker,” Ms. Duke said that she resented the silliness that was forced upon her in the sitcom. In part to distance herself from the TV show, Ms. Duke accepted a role in the raunchy “Valley of the Dolls,” the 1967 film based on the best-selling book by Jacqueline Susann, as Neely O’Hara, a pill-addicted vaudeville star.

Off screen, Ms. Duke suffered crippling bouts of mental illness. She was hospitalized at times and attempted suicide. She found relief only after being diagnosed with and beginning treatment for bipolar disorder in 1982.

Ms. Duke wrote about her illness in the book “A Brilliant Madness: Living with Manic Depressive Illness” (1992), co-authored with Gloria Hochman. While maintaining her acting career, principally on television, she became widely known as an advocate for mental health.

Anna Marie Duke was born in New York City on Dec. 14, 1946. Her father was a cabdriver, and her mother, who suffered from depression, supported her children as a cashier after the couple separated.

Ms. Duke met her agents through her older brother, Ray, whom they also represented. In an autobiography, “Call Me Anna” (1987), co-authored with Kenneth Turan, the actress recalled how the Rosses demanded she take a stage name.

“Anna Marie’s dead,” she said they told her. “You’re Patty now.”

For periods, Ms. Duke lived with the agents.

“I was stripped of my parents, I was stripped of my name, I was eventually stripped of my religion, and they had a blank slate to do with as they wished,” she wrote. “When I was with the Rosses, I quickly went into a kind of limbo, similar to the mind-set of people who are in jail or even mental hospitals. You simply cannot think about the bad things because there is nothing you can do about them.”

Among her earliest TV assignments, at age 8, was an appearance in a dance routine on “The Voice of Firestone.” She polished her diction, eventually learning to speak in a British accent for appearances in television productions including “Wuthering Heights” (1958). (Ms. Duke portrayed a young Cathy, with Richard Burton playing Heathcliff.)

She had amassed dozens of television appearances by the time she appeared on Broadway in 1959 in “The Miracle Worker.”

“Miss Duke, with never an instant’s hesitation, gives us the tortured, infuriated mind within the little child,” theater critic Richard L. Coe wrote in The Washington Post. “The busy little face, the violently active body, the sheer, ghastly loneliness and the triumphant, astonishing breakthrough to the inner mind are expressed in unerring mime. This child’s performance is a marvel.”

Ms. Duke was married at 18 to director Harry G. Falk. After they divorced, she wed rock music promoter Michael Tell but separated from him weeks later. Her subsequent marriage to actor John Astin also ended in divorce.

Survivors include her husband of 30 years, Michael Pearce; a son from Tell who was adopted by Astin, the actor Sean Astin; a son from her third marriage, Mackenzie Astin; a son from her fourth marriage, Kevin Pearce; and several grandchildren.

Even in the throes of her illness, Ms. Duke continued to act. She received Emmy awards for her performances in “My Sweet Charlie” (1970), a TV movie about the relationship between a pregnant white woman from the South and a black lawyer from New York; “Captains and the Kings” (1976), a miniseries about an Irish immigrant; and a 1979 TV movie of “The Miracle Worker” in which Ms. Duke took on the role of Sullivan and Melissa Gilbert played Keller.

She served as Screen Actors Guild president from 1985 to 1988.

“I’ve survived,” she wrote in her autobiography. “I’ve beaten my own bad system and on some days, on most days, that feels like a miracle.”