Sean Astin was with his mother, Anna "Patty Duke" Pearce, when she died on Tuesday morning and he shared her final moments with ET in his only on-camera interview.
"She was surrounded by her family," Astin told ET. "It was in the middle of the night and it was a race to get there. It was a very powerful experience. And then, in her last moments, she was at peace."
WATCH: Sean Astin Remembers Late Mother Patty Duke
Her peace came after a painful battle with sepsis due to a ruptured intestine.
"She had been suffering terribly," Astin said.
Duke died at the age of 69 at a hospital in Coeur D'Alene, Idaho. She was perhaps best known for portraying Helen Keller in 1962's The Miracle Worker, a role that earned her an Academy Award for Best Supporting Actress. At the time, she was the youngest Oscar winner ever at 16.
WATCH: Melissa Gilbert Remembers Patty Duke After Her Death: 'She Was My Second Mom'
Along with her Oscar win, she will also be remembered for helping destigmatize mental illness. After bravely going public about her struggle with manic depression, Duke traveled the nation lobbying as a mental health advocate.
"The more she opened up, the more she shared of her pain. It is very instructive for people," Astin said. "This  bipolar diagnosis was a watershed. 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 former First Lady 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. Duke even opened up to ET about her diagnosis in a 1992 interview.
"I happen to prefer calling my disorder manic depression," Duke told ET at the time. "It is known as bipolar now. And if that works for other people, fine, but I prefer to call it manic depression because it sounds like it felt."
WATCH: Patty Duke Dies at 69 -- Hollywood Pays Tribute
To honor his mother's legacy, Astin has set up a fundraiser for The Patty Duke Mental Health Project, which will continue Duke's campaign of awareness.
"Above all, the way that I think of my mother, the thing that gives me such joy and reverence for her, is that above all else, she was a warrior," Astin said.