The 37-year-old was told by doctors she had an 87% risk of breast cancer and a 50% risk of ovarian cancer.
"Once I knew that this was my reality, I decided to be proactive and to minimise the risk as much I could. I made a decision to have a preventive double mastectomy," she explained.
She finished three months of medical procedures on 27 April.
Writing in The New York Times on Tuesday, the Tomb Raider superstar paid tribute to her "loving and supportive" partner Brad Pitt.
"During that time I have been able to keep this private and to carry on with my work," she said, in an article titled My Medical Choice.
"I am fortunate to have a partner, Brad Pitt, who is so loving and supportive.
"So to anyone who has a wife or girlfriend going through this, know that you are a very important part of the transition. Brad was at the Pink Lotus Breast Centre, where I was treated, for every minute of the surgeries.
"We managed to find moments to laugh together. We knew this was the right thing to do for our family and that it would bring us closer. And it has."
For any woman reading this, I hope it helps you to know you have options.
Meanwhile Pitt has praised his fiancee for her "heroic" decision.
He tells the London Evening Standard: "Having witnessed this decision firsthand, I find Angie's choice, as well as so many others like her, absolutely heroic.
He added: "I thank our medical team for their care and focus. All I want is for her to have a long and healthy life, with myself and our children. This is a happy day for our family."
Jolie, whose mother had cancer and died aged 56, said waking up from the operation can feel "like a scene out of a science-fiction film".
"My chances of developing breast cancer have dropped from 87 percent to under 5 percent. I can tell my children that they don't need to fear they will lose me to breast cancer," she wrote.
"They know that I love them and will do anything to be with them as long as I can."
The mother of six called for women to get themselves checked out.
"I want to encourage every woman, especially if you have a family history of breast or ovarian cancer, to seek out the information and medical experts who can help you through this aspect of your life, and to make your own informed choices."