Asked how important Jolie's support at the premiere was, he says: "I've never not had support from her. I love her, it brings you closer and makes you stronger."
Jolie, whose own mother was 56 when she died of ovarian cancer, underwent weeks of hospital appointments, invasive surgery and recovery during the early months of this year, but the couple are feeling positive.
Pitt's transformation from ladies man to family man is well and truly complete.
Once engaged to Gwyneth Paltrow, he married former Friends actress Jennifer Aniston in 2000, before they split in 2005.
It was rumoured that his relationship with Jolie, whom he met on the set of Mr And Mrs Smith, caused the break-up, and while allegations of adultery were denied from both camps, Pitt and Jolie later admitted they fell in love while working on the film.
The couple are parents to three biological children - Shiloh, 7, and 4-year-old twins Knox and Vivienne - and three adopted children: Maddox, 11, Pax, 9, and 8-year-old Zahara.
Pitt had his children in mind when he signed up for zombie thriller World War Z, he says, particularly the two older boys.
As well as starring in it, he produced it through his production company, Plan B Entertainment.
"These zombies are scary as hell," he says. "And the movie, I believe, works on numerous levels. But primarily, it's complete summer fun and something I wanted to do for my sons to enjoy.
"Some parents have not agreed with me after seeing it, because it's the most intense film that you're going to see all summer! But my kids are savvy and I figure it's called parental guidance ... But yeah, they loved it," he adds.
In World War Z, directed by Marc Forster (Quantum Of Solace) and based on Max Brooks' post-apocalyptic horror novel, Pitt plays former United Nations investigator Gerry Lane, tasked with having to save his family while trying to find, and stop, the source of the epidemic that's turning people into zombies.
"Five years ago, I knew nothing about zombies. Now, I consider myself an expert," Pitt jokes.
"Max's book treats the zombie genre as a global pandemic, spreading much like we've witnessed viruses like Sars travel."
The actor says playing someone normal, not a superhero, appealed. "Gerry can't fly, he can't beat up bad guys. He has no super powers. He's a dad, with a burning need to keep his family safe. To do that, he can only rely on his intellect, his instincts and his experience," Pitt explains.
He has kind words for his on-screen wife Karen, played by Mireille Enos, who's starred in the US TV version of The Killing.
"Mireille is a brilliant partner on set," he says. "She can embody the lovingness of the mother and flip to the ferociousness of the lioness protecting her young in an instant. As in any great relationship, she carried half the weight - a true ally."
As for Forster, who also directed Monster's Ball, Finding Neverland and The Kite Runner, Pitt says: "He can't be pigeonholed as a director; his experience and interest in many genres and types of film is a rarity. His most memorable moments on film are intimate and human.
"It was this quality, juxtaposed against our massive global apocalyptic crisis, that we believed would lead to an unusually authentic and grounded action thriller."
Forster returns the praise, calling Pitt "the best partner".
"We'd never done anything like this and, in that sense, it was a challenge - to work through this genre that was unfamiliar to the both of us, and try and create something fresh and new."
The production woes of World War Z have been closely followed, with Vanity Fair writing that 40 minutes of the film had to be completely re-written and re-shot to fix the ending.
Reports say the budget ballooned to around US$200 million ($250.3 million), while others have speculated that with marketing costs, it may have climbed to as much as US$400 million.
Forster says the difficulty with a blockbuster like World War Z was the sheer volume of extras on set.
"When you're working with a thousand extras and you have Brad Pitt in the middle of it, you can't individually direct all of those people, so it's just more like mass mayhem," he says.
"You give them a general direction and hope it goes right. If it doesn't you have to do it again. It's always much more tricky to pull this off."
Pitt, who became a Hollywood heart-throb after appearing in Thelma And Louise in 1991, and later in films like Meet Joe Black and Fight Club, has received four Oscar nominations in his time, including two for 2011 drama Moneyball.
His forthcoming roles include Steve McQueen's 12 Years A Slave and Ridley Scott's The Counselor and may bring him further acclaim.
But awards don't factor in his game plan.
"I don't play the Oscars game. I had a Golden Globe [for 12 Monkeys] about a decade ago, but they are never the focus," he says. "I believe it's how you personally feel. You want to feel that you've given it your best at the end of the day. We all want to be really good."
Movie profile
Who: Brad Pitt
What: World War Z
When: Opens at cinemas today.
What our reviewer said: In Saturday's 3.5 star review, "Pitt's zombie epic deserves to go viral".
- AAP
