It's a long way from Saskatoon, Saskatchewan, to Carnegie Hall," Joni Mitchell announced as she made her debut at the New York City landmark in 1969. She was wearing a bishop-sleeved blouse under a waistcoat, a long patterned skirt and metallic beads, her waist-length hair and blunt bangs framed epic cheekbones. Twenty-six years old, and less than a decade after leaving the Canadian prairie city she grew up in, the singer-songwriter was already on her way to becoming a style muse.

Mitchell's free-spirited folk-heroine style inspired many of this year's spring collections: Emilio Pucci, Chloé, Dries Van Noten and Valentino among them. Derek Lam got literal, with his models strutting to a Mitchell soundtrack. And her status as poster child of the season was cemented last week when she was unveiled as the Saint Laurent model in a S/S15 campaign shot by the label's creative director Hedi Slimane.

Yet the singular look Mitchell is known for - fresh-faced, ethereal Woodstock flower child - happened almost by accident. Mitchell didn't even attend the Woodstock festival. She only began to pare down her look when David Crosby, whom she met in 1967, encouraged her to "let go of all this elaborate war paint" and embrace his "scrub-faced California culture", as she told Cameron Crowe in a 1979 Rolling Stone interview: "I had just come back from London. That was during the Twiggy-Viva era and, I remember, I wore a lot of make-up... It was a great liberation to get up in the morning and wash your face... and not have to do anything else."

Mitchell seems destined to have become a style idol. As a child growing up in prairie towns (the family eventually settled in Saskatoon when Mitchell was 11) Roberta Joan Anderson, who was always known by her middle name, yearned for glamour. She christened the Simpsons-Sears department store catalogue the "Book of Dreams", thought about becoming a fashion designer and made her own clothes. She did not consider imitation a form of flattery.

"Kids used to copy me and I didn't like it," Mitchell said in a 2005 interview with Reader's Digest. "They would wait until I bought my winter coat and then they would get the same one."

When you're from the Canadian prairies, where the sunsets are infinite and the cold so bitter you feel like you might break into a million ice chips, a bit of glamour is a welcome thrill. Saskatoon is covered by snow for most of winter and exposed skin freezes in seconds, so choosing a winter coat is one of the few big fashion decisions you make every year. So, yes, it sucks when another kid in your class buys the same one.

I, too, spent my formative years in Saskatoon. Although I loved growing up in a somewhat tight-knit community - population 219,056 in 1996 - the city offered little in the way of mystique. Plus, fresh-faced, all-natural, grain-fed prairie girls such as Mitchell, with high cheekbones, beautiful eyes and earthy good looks are a dime a dozen in Saskatchewan. You need to up your style game if you want to stand out.

Mitchell was never a wallflower. "She was, and is, the most original person I know," writes Anne Bayin, Mitchell's childhood friend, via email. "Others copied her. In school, she'd set fashion trends effortlessly by wearing her dad's tie to class. How daring was that? We were still ironing pop decals on our cotton jackets but she'd moved on." Bayin also remembers Mitchell putting sequins on her bedroom walls.

By the time Mitchell reached high school in the "Paris of the Prairies" (one of Saskatoon's more endearing monikers), the self-proclaimed clotheshorse wrote a column for her school newspaper called "Fads and Fashions". Ever fashion-forward, she worked as a shopgirl, occasionally modelling samples for in-store events on weekends. This gave her access to clothes that were "too fashionable for our community", she told Crowe in the Rolling Stone interview.

"She was always very careful about her style," says Tony Simon, who met Mitchell at Nutana Collegiate in Saskatoon. She was in ninth grade, he in grade 11, she was his high school and university graduation date, and they remain close. "She would always be impeccably dressed," he tells me, "even if it was super casual. She was always on the leading edge [of fashion]."

After leaving high school in the early 1960s, Mitchell moved 600km west to Calgary to become a student of Alberta College of Art and Design. Ironically, she made an impression at college by rejecting the look she later became known for.

"She was already singled out as different because she was dressing really nice," says fellow student Jean Grand-Maître, now artistic director of the Alberta Ballet, with whom Mitchell collaborated on the 2007 production of The Fiddle and the Drum. "She loved beautiful clothes but all the students around her were bohemian, so she was an outcast that way." (Today, Mitchell favours Issey Miyake, says Grand-Maître, and owns hundreds of pieces by the Japanese designer.)

John Uren, who hired Mitchell to perform with her ukulele at The Depression Coffee House in Calgary in 1963, remembers her as looking unique. "Joni always looked spectacular. She had a presence about her. She was an art student so it was a case of, I am sure, using her imagination to look incredible."

Ultimately, Mitchell dropped out of art school and left for Toronto before moving to the US and finding the look for which she has become so known. But her style is more mercurial than fashion has allowed. "She has always been the same [with fashion] as her music or her art; she's always searching for something that is meaningful," says Tony Simon. "Whether it's significant because of where it comes from, who designed it or what inspired it, there will be a story to every piece."

In his speech honouring Mitchell at a gala in Los Angeles last year, Crowe called her "the first rock artist to break the anti-fashion mould of the day". In the 1970s, when she performed songs from her album Court and Spark (1974) in a glittery gown, she was told by journalists she had grown "too glamorous for rock and roll".

Perhaps when it comes to the "Joni Mitchell look", it would be best for designers to take a page out of Leonard Cohen's playbook. The fellow Canadian artist, and Mitchell's one-time lover, asked for a moratorium on the use of his classic song "Hallelujah". Maybe the same courtesy should be extended to Mitchell's 1970s style, allowing her to break free finally from the patchwork suede, the floral gossamer gowns, the macramé dresses and the embroidered peasant blouses once and for all.

Let Mitchell finally have her sequins.