At the end of the 1990s, when Diane Warren was the unrivalled queen of the power ballad, her music publisher presented her with a quartet of gold discs and a plaque hailing her as “the career saviour of the 90s”. The discs celebrated the windswept mega-hits Warren had written for Toni Braxton (Un-Break My Heart), LeAnn Rimes (How Do I Live), Celine Dion (Because You Loved Me) and Aerosmith (I Don’t Want to Miss a Thing), the first two of which are still among the bestselling US singles ever.

To be imperial in one pop era is usually to be defined by it for evermore, but Warren has been writing hits for almost four decades, notching up nine US No 1s and 32 Top 10 hits. In 2015, Til It Happens to You, her potent Lady Gaga collaboration for a documentary about campus rape, made her once again the pop equivalent of the striker you turn to when you absolutely have to score a penalty.

Six of her 12 Oscar nominations have come in the past seven years and one of those songs, for the documentary RBG, prompted a thank you note from supreme court judge Ruth Bader Ginsburg herself. Warren provided a theme song for the Biden-Harris campaign called The Change, a feminist anthem for Michelle Obama called This Is for My Girls, and Here’s to the Nights, a lockdown pick-me-up for Ringo Starr with backing vocals by Paul McCartney. “I have two of the Beatles singing on my song,” she says fannishly. “It doesn’t get better than that.”

It’s 8am in Los Angeles and Warren is chugging coffee like she’s filling up a car. Wearing a grey top, a checkerboard scarf and large horn-rimmed glasses, she has a buzzy, fast-talking energy, like the salty best friend in a Nora Ephron movie. We are joined intermittently by her cat Rabbit, who jumps on Warren’s lap, uses her leg as a scratching post, eats paper and so on. “She likes attention,” says the songwriter.

Her owner? Not so much. Growing up in Van Nuys, California, Warren admired songwriters more than performers. “None of it appealed to me,” she says. “I have stage fright. I would hate to be on the road. Some of these artists I work with can’t even walk down the street. I have a great gig because a lot of people don’t know me. I’m like the Wizard of Oz behind the curtain.”

Warren literally has a green curtain behind her, prohibiting any glimpse of her surely fabulous home via Zoom. As soon as we finish talking, she’ll be off to work on a song at “the Cave”, the Hollywood Hills office she has used since 1985. Even though she’s wealthy enough to have bought an entire building in the same street for her Realsongs publishing company, Warren still rents the Cave because she’s a creature of habit. “It’s really dirty and comfortable. I’ve basically lived in there more years than not.”

Warren is “obsessed with writing songs, more so now than ever”. She writes seven days a week when she’s in LA and has never had writer’s block. She does occasionally take a holiday but six days is her limit. Almost uniquely in modern pop, she prefers to work alone. “I don’t know, when there’s 12 writers on a song, what exactly they’re doing,” she says. “Are they getting the coffee? Coming up with a hi-hat pattern in the bridge? I don’t need a writing camp. I am a writing camp.”

At 65, Warren is now releasing her debut album, The Cave Sessions Vol 1. A while back, she took a ballad called Where Is Your Heart to John Legend, who recorded it but didn’t release it. After some back and forth, she figured: why not release it under her own name? Back when she was writing such songs as I Don’t Want to Miss a Thing for Jerry Bruckheimer blockbusters (“girl songs for boy movies”), she would work to a brief, but usually she writes for herself, so that when a request comes in there’s always something in the vault. This means, she says, “Some of my best songs have not been heard yet.”

Warren designed the album – which features previous clients Celine Dion and Paloma Faith alongside newcomers such as G-Eazy and Ty Dolla $ign – to showcase her range. There’s R&B, hip-hop, country and Latin, as well as the expected knockout ballads. While she can carry a tune well enough for a demo, she knows her limits: “The good thing about me singing is that nobody will ever say, ‘Wow, I wonder if I can sing that as good.’”

Every singer she approached said yes, but that is not always the case. When someone turns down a song, does she, as the world’s most successful female songwriter, ever think: “It’s great, don’t be an idiot?” She cackles. “Whenever someone doesn’t get it, I kind of think that. I’m like, ‘I know what I’m talking about, just do the song.’ Sometimes I talk someone into it and I’ve never been wrong.” She mentions Cher, whose initial response to If I Could Turn Back Time was: “I fucking hate it.” Helped by its none-more-80s video (battleship, buttocks), the song ended up rebooting Cher’s career. “A suit might look weird,” says Warren, “but once you try it on, it might be the coolest fucking suit you ever wore. You just didn’t picture yourself in it like I did.”

Warren, whose last romantic relationship ended in 1992, doesn’t claim to pour her own experiences into every song. Her appeal to singers and listeners is the universal imprecision of her lyrics. “I’m not somebody who sits and writes about my life because how fucking boring would that be? My songs are super open. They could go in a million different directions.” The Change, for example, wasn’t political until she gave it to Biden-Harris, and Numb was about losing her mother before the Pet Shop Boys reinterpreted it as post-9/11 anxiety, only to inadvertently return it to the source with a video using old Russian newsreels. “My mom’s family was from Russia,” Warren explains. “It was emotionally devastating watching that video.”

Twilight director Catherine Hardwicke has talked about making a biopic about the songwriter’s early life. As an unruly teenager, Warren was sent to juvenile detention for smoking weed and once stayed with a bunch of heroin addicts when she ran away from home. Nothing grabbed her like songwriting did. “I got kicked out of schools, I hated school, but on my own I would really study the Brill Building writers.”

Her insurance salesman father was right behind her (Dion’s Because You Loved Me was a tribute to him disguised as romance) but her mother was sceptical. “My mom used to say, ‘That song’s really great but take it to Ralphs [supermarket] and see if they’ll give you groceries for it.’ It’s not that she didn’t believe in me, but how do you make a living as a songwriter? It’s a one in a million thing to be successful and to be as successful as I am is one in a billion, probably.”

Warren didn’t get her first sole writing credit, for Debarge’s Rhythm of the Night, until she was 28. Thirty-six years later, she is still delivering the goods. Has she ever felt like she’s cracked pop’s code? She shakes her head. “Every song is cracking the code really. It’s its own solar system. Whatever works in a song only works in that song.”

The challenge of proving herself over and over again is what keeps Warren going and although she would emphatically love an Oscar, she enjoys racking up the nominations. “I am now the only woman in the history of the Academy Awards to be nominated this many times without winning, which I’m kind of proud of. If I could have won one time and then not ever been nominated again, I would definitely choose the longevity game.”

The business of songwriting has changed a lot since she started, though. To avoid plagiarism lawsuits, the credits on some hit songs have as many names as a football squad, expanding to include anyone whose own song sounds vaguely similar. “Uptown Funk got a new writer every week! It’s a strange place we’re in.” Then there’s the emergence of investment funds such as Hipgnosis, which snap up song catalogues for eye-popping sums.

“It’s something I would never do,” she says with finality. “If they’re paying you 20 times what your catalogue is worth, I could see why people would do it, especially if they need the money. I don’t need the money. This is my soul and my soul wouldn’t be for sale at any price.” Could someone feasibly live off the proceeds of just one of her songs? She pauses. “Probably. You could live pretty good on How Do I Live. It depends on how you live, right?”

Warren finds the process of marketing songs these days a drag. “To me it’s like, ‘This is a great song, let’s get it on the radio!’ But it’s like, ‘Well, no, you have to build a story.’ Fuck that. If the Beatles or Prince existed now, I have no idea how that would have worked. They would have had to do TikTok campaigns and if that didn’t work, the label wouldn’t push their music. Who the fuck knows? But it still comes down to an undeniable song. I still believe that.”

I’m guessing that interviews aren’t her favourite activity either. I can sense her itching to get back to the Cave. “I’ve always been about showing up and fucking working,” she says. “It’s one thing to have dreams and aspirations, but without the work it doesn’t happen.” The world’s most successful female songwriter bids me an efficient farewell. “Nice meeting you. More coffee!”

• The Cave Sessions Vol 1 is released on 27 August on Di-Namic Records/BMG.