She then added that her cancer was completely "symptom-free" and that without the mammogram, she never would have known she had cancer as she "never found a lump" and did not feel ill.
"My experience underlines the vital importance of getting screened when you're called in," she shared.
"It's quite extraordinary, 18 months before it wasn't there. So, it had come on from the last mammogram to this mammogram."
As she prepared herself for surgery and an unknown future, the royal revealed her fears of not being able to spend more time with her family, especially her three grandchildren.
"The drive from the Royal Free Hospital, I'll never forget because of course your mind goes into, 'oh my goodness, I've got to have a mastectomy' and you look it up and it's all terrifying, and 'this is what's going to happen' and then, 'I'm not going to see my grandchildren grow up.' That's what goes through your head," she divulged on the show.
It is currently unclear when she was first diagnosed, but an official spokesperson confirmed that her prognosis following the surgery was "good" and that Sarah had been receiving "the best medical care."
The procedure took place sometime in the fourth week of June at King Edward VII Hospital, a private clinic located in central London that has previously treated members of her former royal in-laws, including her late mother-in-law Queen Elizabeth II.
Whilst surgery commonly cures breast cancers, chemotherapy, radiotherapy, and endocrine therapy can assist in reducing the ongoing risk of dying if initial surgeries are not successful.
According to the National Breast Cancer Foundation, approximately 57 Australians are diagnosed each and every day, equating to 20,000 per year or one in seven women being diagnosed in their lifetime.
In 2022 alone, approximately 3,200 Australians passed away from breast cancer, the equivalent of nine deaths per day.
Shortly after news of her surgery became public, the Duchess spoke candidly about her breast cancer journey on her podcast Tea Talks, revealing the specific surgery she had.
"We're taping the podcast today, but tomorrow I'm going in for a mastectomy, a single mastectomy, it's very important that I speak about it," she said.
"I want every single person that is listening to this podcast to go and get checked, to go get screened."
She then went on to explain that it was "easy to put off" getting a routine breast exam and was beyond thankful that her friend had reminded her to go to the doctor.
"I'm taking this as a real gift to change my life," she added.
"It's vital that we wake everyone up to go get screened. Not just breast cancer, all the checks."
It's been a busy few months for the Duchess after she became a grandmother for the third time with the arrival of baby Ernest George Ronnie Brooksbank on the 30th of May.
The bouncing baby boy is the second child of her youngest daughter Princess Eugenie and her husband Jack Brooksbank, who also share toddler August Philip Hawke, who was born in February 2021.
Her eldest daughter Princess Beatrice welcomed her first child into the world - Sienna Elizabeth Mapelli Mozzi - with her husband Edoardo Mapelli Mozzi on the 18th of September 2021.