EXCLUSIVE: Sean Astin remembers late mother Patty Duke: 'She was a warrior'

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EXCLUSIVE: Sean Astin Emotionally Pays Tribute to the Late Patty Duke: 'My Mother Was a Warrior'

Sean Astin spoke with ET on Tuesday about his late mother, actress Anna "Patty Duke" Pearce, who died Tuesday morning at age 69.

Astin said that Duke was "surrounded by her family" prior to her death, after relatives traveled to her hospital bedside in Coeur D'Alene, Idaho. The 45-year-old actor described being with his mother in her final moments as "a powerful experience."

"She was at peace," he recalled. "When it was over, we all looked at each other and we thought, you can be an atheist, a skeptic, agnostic, you can be whatever you want, but what just happened in this room is indescribably beautiful."

WATCH: Anna 'Patty Duke' Pearce, Oscar-Winning Actress, Dies at 69

Duke became a major advocate for mental health awareness after being diagnosed with bipolar disorder in 1982, a moment Astin described as a "watershed" for his mother's life and legacy.

See photos of Patty Duke throughout her career:

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Patty Duke through the years
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EXCLUSIVE: Sean Astin remembers late mother Patty Duke: 'She was a warrior'
Child actress Patty Duke roller skates in Central Park in New York City, April 6, 1961. (AP Photo)
Gregory Peck, holding pocket watch, jokes with, from left, Patty Duke, Joan Crawford and Ed Begley backstage at the Academy Awards at Santa Monica Civic Auditorium in Los Angeles, Ca., April 8, 1963. Peck won best actor for "To Kill a Mockingbird"; Duke won best supporting actress for "The Miracle Worker"; Crawford accepted the Oscar for Anne Bancroft, who won best actress for "The Miracle Worker"; and Begley won best supporting actor for "Sweet Bird of Youth." (AP Photo)
Child actress Patty Duke is filling out the letters of her name as it appears on the marquee at the Playhouse Theatre in New York City, advertising the play "The Miracle Worker," on March 25, 1960. 13-year-old Patty, who stars opposite Anne Bancroft in the hit show, is held and supported by Mannie Kay, president of the sign company. (AP Photo)
Actress Patty Duke, star of ABC-TV's "The Patty Duke Show," and Colette Daiute, right, who recently won the Miss Teenage America contest, meet at a Hollywood beauty salon as they prepare for their roles on Patty's show, Dec. 3, 1965. Fixing Patty's hair is hair stylist Jeanne Bodel. (AP Photo)
Winners of the best supporting performers Oscars pose with the presenters at the annual Academy Awards at Santa Monica, Ca., on April 8, 1963. Left to right; presenter George Chakiris, Patty Duke who won the best supporting actress for her role in The Miracle Worker, Ed Begley, names best supporting actor for the Sweet Bird of Youth, and Rita Moreno who presented Begley with his Oscar. (AP Photo)
UNITED STATES - NOVEMBER 12: JOURNEY TO THE UNKNOWN - 'Last Visitor' 6/16/69 Patty Duke (Photo by ABC Photo Archives/ABC via Getty Images)
UNITED STATES - NOVEMBER 12: JOURNEY TO THE UNKNOWN - 'Last Visitor' 6/16/69 Patty Duke (Photo by ABC Photo Archives/ABC via Getty Images)
LOS ANGELES, CA - FEBRUARY 2: Patty Duke attends 27th Annual Golden Globe Awards on February 2, 1970 at the Ambassador Hotel in Los Angeles, California. (Photo by Ron Galella, Ltd./WireImage)
LOS ANGELES, CA - FEBRUARY 2: Patty Duke attends 27th Annual Golden Globe Awards on February 2, 1970 at the Ambassador Hotel in Los Angeles, California. (Photo by Ron Galella, Ltd./WireImage)
Patty Duke during 27th Annual Golden Globe Awards at Ambassador Hotel in Los Angeles, California, United States. (Photo by Ron Galella/WireImage)
Actress Patty Duke appears as a witness before a House Appropriations Subcomittee holding hearings on funding for mental health programs on Capitol Hill in Washington, D.C., April 27, 1989. At left is chairman of the panel Rep. William Natcher of Kentucky. (AP Photo/Dennis Cook)
CANADA - JULY 25: Patty Duke's childhood of sorrows. 'Perfect teen' image hid tale of exploitation and abouse/H1 (Photo by Andrew Stawicki/Toronto Star via Getty Images)
John Astin and Patty Duke during 'Hedda Gabler' Los Angeles Premiere at Huntington Hartford Theater in Hollywood, California, United States. (Photo by Ron Galella/WireImage)
MARCUS WELBY, M.D. - 'Unindicted Wife' - Aired on February 25, 1975. (Photo by ABC Photo Archives/ABC via Getty Images) JOHN ASTIN;PATTY DUKE
FILE - In this Aug. 17, 2004 file photo, actress Patty Duke is honored with a star on the Hollywood Walk of Fame in Los Angeles.Duke, who won an Oscar as a child at the start of an acting career that continued through her adulthood, died Tuesday, March 29, 2016, of sepsis from a ruptured intestine. She was 69. (AP Photo/Damian Dovarganes, File)
Award-winning actress Patty Duke poses for photographers with her granddaughters, 2-year-old Elizabeth Astin (L) and 7-year-old Alexandra Astin following an unveiling ceremony, honoring Duke with the 2,260th star on the Hollywood Walk of Fame in Los Angeles, California August, 17, 2004. Duke won the Oscar for best supporting actress for "The Miracle Worker" at 16, the youngest at the time to be so named. REUTERS/Jim Ruymen JR/GN
HOLLYWOOD - AUGUST 17: Actor David Ogden Stiers kisses actress Patty Duke at the ceremony honoring Duke with a star on the Hollywood Walk of Fame August 17, 2004 in Hollywood, California. (Photo by Vince Bucci/Getty Images)
Award-winning actress Patty Duke (C) poses with sons, MacKenzie (L) and Sean Astin following an unveiling ceremony honoring Duke with the 2,260th star on the Hollywood Walk of Fame in Los Angeles, California August, 17, 2004. Duke won the Oscar for best supporting actress for "The Miracle Worker," at 16, the youngest at the time to be so named. REUTERS/Jim Ruymen JR
Patty Duke during Patty Duke Honored with a Star on the Hollywood Walk of Fame for Her Achievements in Film at 7000 Hollywood Blvd. in Hollywood, California, United States. (Photo by Jesse Grant/WireImage)
Actress Patty Duke is seen during an interview in New York, Nov. 18, 2002. Duke, Helen Keller in "The Miracle Worker," 1960s TV icon and pill-popping heroine of "Valley of the Dolls," has joined the cast of the Broadway revival of "Oklahoma!" (AP Photo/Louis Lanzano)
Academy and Emmy award-winning actress, Patty Duke is shown during a news conference at the Paley Center for Media in Beverly Hills, Calif., on Tuesday, March 23, 2010. The cast of the 1960s TV comedy "The Patty Duke Show," are promoting the Social Security Administration's service allowing baby boomers to apply online for retirement and Medicare benefits. (AP Photo/Damian Dovarganes)
The cast of the television hit 1960s sitcom: âThe Patty Duke Show," Paul O'Keefe, left, and Patty Duke, smile during a Social Security news conference at the Paley Center for Media in Beverly Hills, Calif., on Tuesday, March 23, 2010. The cast of the 1960s TV comedy now as seasoned citizens, are promoting the Social Security Administration's service allowing baby boomers to apply online for retirement and Medicare benefits. (AP Photo/Damian Dovarganes)
Patty Duke speaks at the national premiere of her new Social Security Public Service Snnouncement at the Paley Center for Media in New York City. January 6, 2009 Credit: Dennis Van Tine/MediaPunch/IPX
HOLLYWOOD, CA - MARCH 7: Actress Patty Duke Astin receives her award for Favorite Dual-Role on stage at the 2nd Annual TV Land Awards held on March 7, 2004 at The Hollywood Palladium, in Hollywood, California. (Photo by Kevin Winter/Getty Images)
Actress Patty Duke talks about working on a student production of 'The Four Children of Tander Welch,' in Columbia, South Carolina, on November 20, 2006. (Photo by Gerry Melendez/The State/MCT via Getty Images)
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"All of a sudden, [here] was this thing that, not only was it important for her own life, because now she could design a caregiving plan, and she could get medication, and there was a name for what her series of symptoms was," he explained. "But now, she had a new identity and a whole new mission. A whole new sense of purpose, and that was to share what she had gone through with other people."

The actress, along with other advocates like Rosalynn Carter and Mariel Hemingway, spoke out publicly about her bipolar disorder and other psychiatric health issues at a time when mental illnesses carried an even heavier stigma.

"She was one of the first public figures with a big public profile who did that," Astin recalled. "What's so funny is, as her son, seeing her -- the real her, the authentic her -- which most people saw as well... the more she opened up, the more she shared of her pain, the more she revealed how excruciating pain can be turned into bliss."

"She just chartered a path," he continued. "She would do fundraisers and she would lobby Congress and write another book specifically on bipolar disorder...She was indefatigable. Wherever I go on this planet, for my whole life, people would come up to me and they'd say to me, 'How's your mom?'"

WATCH: Melissa Gilbert Remembers Patty Duke After Her Death: 'She Was My Second Mom'

Astin reposted his family's statement on his mother's death on Tuesday, adding the simple caption, "I love you, Mom." He also made sure to let friends, followers and fans know that Duke's legacy of compassion and awareness will endure long after her death, posting a link to The Patty Duke Mental Health Project.

I love you mom.OUR FAMILY STATEMENT ON THE PASSING OF PATTY DUKEThis morning, our beloved wife, mother, grandmother,...

Posted by Sean Astin on Tuesday, March 29, 2016

"The story of her life is, there was always something challenging. Some physical malady, some mental thing, some financial thing, some career thing, some something," Astin said of his mother. "The word I use to describe my mom when I write about her, when I talk about her, she was a warrior. My mom was a warrior in the grand tradition of the bravest, strongest warriors from ancient Greco-Roman history. My mother was a warrior."

WATCH: Patty Duke Dies at 69 -- Hollywood Pays Tribute

Oscar-Winning Actress Anna 'Patty Duke' Pearce Dies at 69


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