Los Angeles, California - The Newsroom star Olivia Munn revealed Wednesday that she has undergone a double mastectomy after being diagnosed with breast cancer.

Munn took to Instagram to tell followers her news, just days after she attended the Oscars ceremony with her partner, comedian John Mulaney.

The 43-year-old said the cancer had been discovered almost a year ago after her doctor noted she was at elevated risk for the disease and suggested she undergo an additional scan, 12 months before a routine screening.

"I wouldn't have found my cancer for another year – at my next scheduled mammogram," she told followers.

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After initial tests, a biopsy was ordered.

"The biopsy showed I had Luminal B cancer in both breasts. Luminal B is an aggressive, fast moving cancer," she wrote, adding that the double mastectomy was performed 30 days later.

"I went from feeling completely fine one day, to waking up in a hospital bed after a 10-hour surgery the next," she wrote in a post, with photos taken apparently during her treatment.

"I'm lucky. We caught it with enough time that I had options. I want the same for any woman who might have to face this one day."

Munn walked the red carpet at Sunday's Academy Awards ceremony in Hollywood, where her hubby Mulaney – whom she shares a child with – presented an Oscar, and later attended the celebrity-filled Vanity Fair after party.