The couple looked happy and relaxed as they stopped to meet fans and pose for photos before the premiere of Pitt's new film, World War Z.
Jolie, who made her announcement last month, waved and smiled for fans, telling reporters: "I feel great. I feel wonderful."
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Appearing close to tears at times, she thanked her fiance and family for their support. Pitt himself was full of emotion as he spoke about his fiancee's health.
"He's been extraordinary," said Jolie, wearing a long black dress for the premiere. "He's such a wonderful man and a wonderful father and I'm very, very lucky.
"I'm very happy just to see the discussion about women's health expand and that means the world to me. After losing my mom to these issues, I'm very grateful for it, and I've been very moved by the support from people. I'm really very grateful for it."
Brad said of his fiancee: "She is doing great, it is business as usual, it is the next stage for us."
Asked how important it was to have her support at the premiere he said: "I've never not had support from her. I love her, it just brings you closer and makes you stronger."
Jolie stopped briefly on the red carpet to hold the face of one fan overcome with tears, and took a photo with her.
World War Z stars Pitt as Gerry Lane, a UN employee in a frantic race across the globe to battle the zombie pandemic that is taking over the world.
The couple were joined on the red carpet in Leicester Square by Pitt's co-stars Mereille Enos, Daniella Kertesz and Ludi Boeken, director Marc Foster and producers Dede Gardner and Jeremy Kleiner.
Jolie opted to have the double mastectomy after finding out she carried a faulty gene, called BRCA1, which made her likelihood of developing breast cancer 87%. The operation cut that risk to just 5%.
The Salt actress's mother, Marcheline Bertrand, died of ovarian cancer, also associated with the gene, in 2007.
Her aunt Debbie Martin died of breast cancer just a week ago and Jolie missed the funeral to be at Sunday night's premiere.
World War Z is released on June 21.