Scott was found dead in her Manhattan apartment at 10 a.m. Monday; no note was found and there was no sign of foul play, police said. The designer had texted her assistant 90 minutes earlier and asked her to come to her apartment but didn't say why. She was found kneeling with a scarf wrapped around her neck that had been tied to the handle of a French door, police said.
Her spokesperson requested privacy for her family and friends. Just last month Scott, who was believed to be 49 but had not disclosed her precise age, canceled her London Fashion Week show, due to reported production delays.
Jagger's representative said the singer was "completely shocked and devastated by the news" of her death.
Scott, whose elegant designs in lush fabrics were favored by celebrities like Madonna, Nicole Kidman, Oprah Winfrey, Penelope Cruz and first lady Michelle Obama, was a fixture on Jagger's arm since she met the Rolling Stones frontman in 2001. On red carpets, the striking 6-foot-3 designer towered over her famous 5-foot-10 boyfriend.
In 2006, five years after they became a couple, Scott founded her eponymous label, with an initial collection based on the "Little Black Dress." She became known for designs that had a vintage feel and bared little skin, like her famous "headmistress" dress - prim, with three-quarter sleeves, but also close-fitting and stylish.
Madonna was one of those who wore the dress. "This is a horrible and tragic loss," the singer said in a statement released by her publicist. "I'm so upset. I loved L'Wren's work and she was always so generous with me."
Anna Wintour, editor in chief of Vogue, called Scott "a total perfectionist, someone who absolutely embodied everything her marvelous clothes stood for: strength of character combined with a confident and powerful style. In person, L'Wren was always unbelievably generous, gracious, kind, and so much fun. Her old world American manners and charm were from another time, but her sensibility was always fiercely modern."
And supermodel Naomi Campbell, a close friend, wrote on WhoSay that Scott was "the epitome of elegance and femininity yet still had a girlish quality. I will miss her honesty and I will miss her friendship. My heart goes out to Mick and all who loved her and were loved by her."
In 2009 Scott introduced a shoe collection, and in 2010 she collaborated with Lancome on a makeup line and a fragrance. In 2011 came a handbag line, in 2012 an eyewear collection, and late last year, a collaboration with Banana Republic for a line of affordable clothes.
Though her studio is based in London, Scott presented her runway shows in New York until recently. They were exclusive A-list affairs like few others. In Februar
y 2012, for example, the designer welcomed guests into the wood-paneled, chandeliered banquet hall of an Edwardian building in Chelsea. Guests were offered white wine in tall glasses as they entered, then were seated at a long table. Before them were plates of caviar, served with a baked potato and sour cream. Fiddling with the lighting and the technical details was none other than Jagger, who also stood next to Scott during post-show interviews.
Adding to the sense of luxury, Scott was known to send huge bouquets of roses and handwritten notes of thanks to reporters afterward.
Her clothes were luxurious, too, making ample use of velvet and satin. There were bolero jackets and tea-length dresses, long capes - lined in feathers, perhaps - and high-waisted pencil skirts.
Scott's designs were "very (much) based on her own personal style ... a very interesting style that combined the strict and the sexy," said Valerie Steele, director of The Museum at the Fashion Institute of Technology in New York. "Not sexiness like body exposure, but sexiness like a very strict governess. They tended to be covered up yet form-fitting and beautifully constructed, beautifully made." Steele said Scott's clothes were "were more sophisticated than the average red carpet gown" and added that Scott "had a very precise vision of what she wanted them to look like."
Scott was adopted by Mormon parents and raised in Roy, Utah, which had a population of less than 10,000 at the time.
As a teenager, she developed a love of clothes and made her own on the sewing machine, according to biographical notes from London Fashion Week. She made her way to Paris after high school where, aided by her height and striking looks, she found work as a model for some prominent photographers.
But she became more interested in working with clothes than modeling them, and eventually made her name as a top stylist in Los Angeles and also a costume designer for films like "Ocean's 13."
Scott also designed a huge wardrobe for boyfriend Jagger to wear during the Rolling Stones' "50 and Counting" anniversary tour. The band is currently on its "14 On Fire" tour, scheduled to play six concerts in Australia beginning Wednesday in Perth, according to the RolingStones.com website.
Among the stars who wore Scott's designs was actress Olivia Wilde.
"L'Wren Scott was brilliant, elegant, kind, and generous," Wilde wrote on Twitter. "What a tragedy."
Scott is survived by a brother, Randall Bambrough of Ogden, Utah, who declined comment.
Associated Press writers Mesfin Fekadu and Leanne Italie contributed to this report.