When Lauren Weisberger was 22 years old, she got the job that she would end up being famous for all her life. Having “literally never had a job beyond working in a frozen yoghurt place, with a bit of babysitting and lifeguarding”, but hoping to become a writer, she secured an interview to be assistant to Anna Wintour, then editor-in-chief of American Vogue. To the general horror of the Vogue office in New York, she arrived carrying her CV and writing samples in the sensible, dowdy leather briefcase her parents had given her as a graduation present. “I was fresh out of college – four years of studying and partying and wearing sweatpants all day long. I wasn’t into fashion, so Vogue was a completely foreign culture to me.” To everyone’s surprise, she got the nod.

The role was “demanding and fast-paced and stressful and head-spinningly around-the-clock. I would wake up every morning to voicemails left overnight, endless tasks that all needed to be done right away.” This was 1999 and “Anna didn’t have a computer. As her assistants, we were her computers. And everyone in the office was tall and gorgeous and slender and totally obsessed with this world in a way that felt toxic to me. Every minute in that office felt like an emergency. I was just a kid. I went into survival mode.”

Twenty-five years later, the lobby of the smart London hotel where Weisberger is staying has a vast red stiletto shoe with a pitchfork for a heel in its lobby. She turned that first job of hers into a fictionalised novel, which became a movie, which became a cultural phenomenon. It is now a splashy West End musical, scored by Elton John and scheduled to open on 1 December. When we speak, Weisberger has just attended a preview, watching Georgie Buckland as Andy, the story’s fictional assistant, walking on stage (with the briefcase, which got a big laugh) and bringing the house down with the story that starts with that first job.

Today, in a black sweater and dark jeans, discreet sparkling stud earrings and an expensive-looking white coat, Weisberger looks successful, chic, sophisticated. She still doesn’t care for fashion, she says, brushing off compliments on her clothes: “I just wear black because it’s easy.” But her life is entwined, still, with the boss her bestseller helped to turn into a legend. She owes her perfectly waved long blond bob to Wintour. “One day soon after I started, she looked at me and she was like, ‘Lauren, your hair, I can’t even –’” Weisberger mimes a shoo-ing gesture of displeasure, like someone who has just seen a spider in the sink and requires it removed. “She sent me to a proper New York salon to get a colour and cut. And that’s where I still go. They are my hairstylists, and my dear friends, to this day.” Her hair looks great. Say what you like about Wintour, she knows her stuff.

Weisberger has had a career most writers fighting to make ends meet in the hardscrabble world of fiction can only dream of. While none of her subsequent seven novels have matched the success of her debut, she makes a good living and has a comfortable home in Connecticut with her family. But it hasn’t been a fairytale. It is hard to remember, given its current momentum, the furore The Devil Wears Prada caused on publication in 2003. The New York Times called it “bite-the-boss fiction”, slamming its protagonist for having “an unbecoming superiority complex [that] is one of the main problems of this callow book”.

Weisberger says the reaction “left me traumatised. Powerful women, journalists who I respect to this day, were offended by it. They felt I had not paid my dues, that I was whining and complaining about having to get to work early and get coffees. The response was essentially: who does she think she is? All this noise did wonderful things for book sales, but had I known what would happen, I would not have written the book. I had not understood that Anna would ever know or care about the book, or that anyone in the media would be remotely interested. Of course, from where I am now, I know the book has allowed me to do what I love more than anything, which is to spend my career writing. But at the time, if I had had the chance to take it all back, I would have like a shot.”

The really funny thing, she says, is that she never actually intended to write the book. “I didn’t think of it for a second – while I had the job.” After she left, she enrolled in a writing class, where “everyone else was a grownup writing first-person accounts of really serious things like divorce and drug use. I had nothing like that to share, but what I did have was this crazy experience that I hadn’t had time to process. The book I wrote was not a takedown – it was a fun romp about my experience. I came up with the title in class, off the cuff.”

Unbeknown to Weisberger, her publishers fanned the flames of gossip, sending out proof copies wrapped in plain brown paper to every assistant at Condé Nast, owners of Vogue. Further brouhaha was whipped up by the fact that the publisher, wary of the wrath of Condé Nast, issued nonsensical denials that the Miranda Priestly character was based on Anna Wintour. “That seems laughable now, but at the time they were like, ‘Don’t get us sued.’”

Weisberger has not heard from Wintour since she stopped being her assistant. “I don’t think I’m even a blip on her radar,” she shrugs. “But who knows. She’s an enigma.” Wintour did see the film on its release in 2006: she famously wore Prada to the screening, suggesting that the devil does at least have a sense of humour. But she has never commented on it, nor on the book that turned her into a household name.

It seems to me Weisberger has done more than anyone – besides Wintour herself – to build the legend that exists around a Vogue editor who has become an icon of female power. But Weisberger demurs. “Oh no, not just me. Two words – Meryl Streep. She did an unbelievable job. When the film was being made, I was like, ‘Oh, this is fun. It’s cool to watch a movie get made and maybe some people will go see it.’ But Meryl elevated the Miranda Priestly character to another level with her genius. Miranda is pretty one-dimensional in the book: tough, cold, incredibly competent, at the top of her game. But Meryl Streep doesn’t do one dimensional. She brought Miranda to life.”

The Devil Wears Prada is not just about Wintour, though, or Vogue, or fashion, or New York. “A lot of people connect with it as a story of a terrifying boss,” says Weisberger. “We’ve all had an experience like that.”

And it tapped into generational conflict in the workplace, at a time when the boomer generation still ruled with a rod of iron. Rereading it now, the mood of rebellion against unreasonable demands – yelled from corner offices – feels like a prescient foretelling of the way generation Z are now demanding a reset of office culture and the work-life balance.

“It was a different time,” says Weisberger. “It was like, ‘Shut your mouth, pay your dues, suck it up.’” The book is also pin sharp on society’s myopic view of beauty. Andy is body-shamed in the Vogue canteen for the crime of ordering a soup with cream in it, while a co-worker who wakes up feeling unable to meet the Vogue office’s unwritten dress code of stick-thin glamour considers “calling in fat”. Says Weisberger: “It felt very normal then. It was accentuated at Vogue, certainly, but it was much broader than that. It was in the culture, in the way I was raised. Thin was all that mattered. We’ve made progress there, I think. I hope. There is definitely still a premium placed on being thin, but at least we are learning to check ourselves.”

As a West End musical, The Devil Wears Prada – first a scandalous book, then a darkly comic movie – gets a feelgood third act. Vanessa Williams, playing Priestly, rises on to the stage through a trapdoor in sunglasses to rapturous applause: Nuclear Wintour, but with a sugary sprinkle of Taylor Swift or Beyoncé in stadium mode. Assistants sashay about arm in arm, hair-tossing like supermodels on a Versace catwalk. Andy gets a Chanel makeover. After an aborted first run in Chicago two years ago (“It just wasn’t ready to go to Broadway and I didn’t know what would happen, didn’t know if the project was going to die at that point”) the show has had, in modern fashion parlance, a glow-up. More jokes, more energy, more of the subversive irreverence that made the book a hit – and much better clothes. “It’s fun, isn’t it?” says Weisberger. “And joyful. We need joyful right now.”

Back when she was being yelled at for not getting Wintour’s coffee order fast enough, or for sending the wrong flowers to Donatella, Weisberger would call home and her mum would tell her to hand in her notice. “But my father would say, ‘This is an incredible opportunity, a bird’s eye view no one your age gets.’ I wish now that I’d had the emotional fortitude to have more perspective, because I could have learned a lot more. Because, aside from all the noise about Anna, she is remarkable, the very best at what she does. And I didn’t fully appreciate that then. But if I had done, I don’t know that I would have been able to poke fun at it. And then none of this would have happened.”