"You never know what's going on in someone's life ... privately, people are battling things that you'd never know," Munn told "Good Morning America" co-anchor Michael Strahan.
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Munn said she is speaking out and sharing more of her personal experience in the hopes her story will help others.
The actress first opened up about her private health battle on social media in March and revealed at the time that she had been unexpectedly diagnosed with breast cancer last year.
Munn told Strahan when she first heard the word "cancer," she immediately thought of her son Malcolm, whom she shares with her partner, comedian John Mulaney.
"I mean, honestly, I just thought of my baby," Munn said. "You know, cancer is the -- that's the word you don't wanna hear. There's a lot of other things that you feel like you can beat. But you know, cancer takes down a lot of people. And I just thought about my baby."
Munn said she was diagnosed with stage 1 aggressive Luminal B breast cancer in both breasts in April 2023.
"They said that if I was extremely aggressive, that I could fight this and win," Munn said. But, at the same time, once they started finding all of these little spots in my breasts like, 'Oh, there's another tumor. There's another tumor,' there was such an urgency to it because we had to get in and make some really big decisions."
Undergoing treatment
Munn revealed in March that she underwent a double mastectomy just 30 days after receiving her breast cancer diagnosis. She also shared an emotional, raw moment of herself on video with her doctor.
In the clip, her doctor tells Munn, "Do it for him, do it for your baby," before giving her a hug. Then, Munn is captured on camera saying, "I'm ready."
Munn told Strahan that in that moment, she was unsure whether or not she was actually ready for surgery.
"I just had to say it. You know, sometimes ... you just say it and hope that your body will go with it," the 43-year-old said.
Receiving support
Munn also said her partner John Mulaney has been one of her biggest supporters along her journey, making her laugh as she faces enormous challenges.
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"He's honestly just the best human being. He comes to everything in life with so much compassion and understanding," she said.
"But the one problem is that he makes me laugh so much, there's times I'm like, 'You have to leave the room,'" she added. "I just say, 'Leave the room. You have to leave. You're gonna make me angry now because ... I'm going to, you know, literally pop a stitch.'"
Confronting fertility and the future
Munn opened up in the interview about her decision to undergo egg retrieval, something she said she had done twice before in her 30s. Munn said she and Mulaney hope to have another baby in the future.
"John and I had a long talk about it. We realized that we weren't done growing our family," Munn said. "So right after the double mastectomy, I went through a round of egg retrieval and that's a scary process because I have a cancer that feeds off of hormones so I knew there was a risk."
"And our doctor said, 'Look, we're gonna get one for you and then we're gonna call it,'" she continued. "And then our doctor called and he said, 'Hey, we got the results back. It's two healthy embryos.' And I, we, I mean, just started bawling, crying, both of us."
In addition to egg retrieval, Munn also discussed what her cancer treatment has been like.
Luminal B, the type of breast cancer Munn has, is hormone-positive. Munn's treatment involves suppressing the production of the hormones that cause the cancer cells to grow and can mimic symptoms of menopause.
"The hormone suppression therapy is brutal and it gave me next level, debilitating exhaustion," Munn said. "I was just in bed all day long, all day long. My quality of life was so minimal and I wasn't able to be there for my baby."
Munn said her son Malcolm also noticed her change over the months, something she said motivated her to undergo another type of treatment -- a hysterectomy and oophorectomy, which includes the removal of the uterus, fallopian tubes and ovaries. Munn said she underwent this latest treatment in April, making it her fifth surgery since her breast cancer diagnosis.
"Whenever Malcolm would come into the home, he'd run straight to my bed because that's where he knows I am, like, that's what he associated with me," she said. "And that was just too difficult for me to take. I had to find out if there was another option. So I opted to do the hysterectomy with the oophorectomy. And almost immediately after that, my energy just came back full force."
After her multiple surgeries and treatments, Munn said her doctor wants her to start yet another treatment -- a medication that could potentially stop her body from producing cancer-growing hormones elsewhere.
"I have been so aggressive. It was, you know, I did everything they told me to do. I did all the big surgeries. And now I'm saying, 'Do I have to do this extra drug?'" Munn recalled of her reaction, adding that all of her treatments have been "so tiring."
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But Munn also said she is planning on continuing further treatment.
"I know that I'm gonna stay aggressive. I know I'm going to do it. I'm just surprised that I'm even asking for anything less than aggressive because, I mean ... it just feels nonstop," Munn said.
Reflecting on the past year
Throughout her journey this past year, Munn said she has learned more about herself and about what she values in life.
"I've learned that I'm a lot braver than I thought I was. And I learned that the most important thing to me in life is my family," she said. "Like, everything else can go away. I don't have my career, I don't have my body the way that it looked before. But as long as the people that I love and care about are here and healthy and thriving, nothing else matters."
Why Munn is sharing her story
Munn said her decision to share her experience on social media has been driven in part by her young son.
"Because if I didn't make it, I wanted my son, when he got older, to know that I fought to be here, that I tried my best," the mom of one said.
"You want the people in your life, you want the people that maybe don't understand what's going on right now, to know that, like, you did everything you could to be here," she added.
Since sharing her story widely, Munn also said the outpouring of support from the public has put a smile on her face and that the whole experience has led her to be kinder to herself.
To watch more of Michael Strahan's exclusive ABC News interview with Olivia Munn, tune in to "Nightline" on May 16.