"I started with the breasts, as my risk of breast cancer is higher than my risk of ovarian cancer, and the surgery is more complex," Jolie wrote.
She said that on April 27 she completed the three months of medical procedures that the mastectomies involved.
Jolie said her chances of developing breast cancer are now down to five percent.
Jolie and Pitt have three adopted and three biological children.
"I can tell my children they don't need to fear they will lose me to breast cancer," Jolie said.
Jolie described a two-stage surgical process, the second of which is an operation that can take up to eight hours as the breast tissue is removed and temporary fillers are put in place.
"You wake up with drain tubes and expanders in your breasts. It does feel like a scene out of a science fiction film. But days after surgery you can be back to a normal life," Jolie wrote.
Jolie said Pitt has been a huge source of support -- "we managed to find moments to laugh together," she said -- and she has only small scars after the ordeal, with nothing alarming for her children to see.