Mick Jagger of the Rolling Stones arrives with stylist L’Wren Scott in 2007 for the U.K. premiere of Martin Scorsese's film “Shine a Light.” (Daniel Deme/European Pressphoto Agency)

L’Wren Scott, a high-fashion model turned Hollywood stylist then chic clothing designer who had a long romance with Rolling Stones lead singer Mick Jagger, was found dead March 17 at her Manhattan apartment. She had long-shrouded her age but was reported to be 49.

The Associated Press reported that Ms. Scott’s assistant found her kneeling with a scarf wrapped around her neck that had been tied to the handle of a French door. New York media outlets said the death is being investigated as an apparent suicide.

A statuesque 6-foot-3 with 42-inch legs, Ms. Scott did modeling work in Paris before establishing herself in Hollywood as a stylist in the 1990s and early 2000s. She dressed stars such as Nicole Kidman, Sarah Jessica Parker, Angelina Jolie, Penelope Cruz and Julianne Moore, and created outfits for Oprah Winfrey and former French first lady Carla Bruni-Sarkozy.

In 2006, she debuted her initial fashion collection, Little Black Dress, after noticing a lack of tailored, sophisticated and sexy “LBD’s” in her famous clients’ closets.

“As a woman, a little black dress means a clear point of view,” Ms. Scott said in a contribution to Vogue editor Andre Leon Talley’s book dedicated to the little black dress. “As a designer, it means business and what every woman needs. It represents what every woman wants: confidence.”

She was best known for her luxurious, elegant, red-carpet designs; examples include Kidman’s frothy, ivory feathered ensemble for the 2009 Oscars; Cruz’s red, silk, hand-embroidered sequined dress for the 2011 Oscars; and Amy Adams’s shimmering cap-sleeved, midnight-blue column gown, also for the 2011 Oscars.

Her contemporary designs were distinctively feminine and highlighted the female form. “A man only understands fashion if it has a waistline,” Ms. Scott quipped at a fashion awards ceremony in 2012.

“Even though Scott was inspired by her own willowy frame, women with significant curves and confidence, such as first lady Michelle Obama, embraced her designs,” fashion critic Robin Givhan wrote in The Washington Post. “The clothes weren’t easy, but they were welcoming.”

Ms. Scott had shown her collections in New York, Paris and London. Last month, Ms. Scott pulled out of London’s Fashion Week show, attributing the last-minute cancellation to production problems. British newspapers reported that her company, LS Fashion Ltd., had financial problems.

Beyond her clothing line, she had a handbag, shoe and eyewear collection, launched a beauty line and fragrance with Lancôme, and debuted a jewelry line with Vera Wang and DeBeers.

Most recently, she collaborated on a makeup collection with makeup artist Bobbi Brown, introduced a women’s collection for the clothing store Banana Republic and designed stage outfits for Jagger for the Rolling Stones’ 50th-anniversary tour.

The couple reportedly met at a fashion photo shoot in Paris in 2001, where she had been booked as a stylist. Ms. Scott towered over Jagger, who was at least 20 years her senior. They were together for 13 years — longer than Jagger’s two marriages.

Jagger, who is presently on tour in Perth, Australia, released a statement on his Facebook page Tuesday morning: “I am still struggling to understand how my lover and best friend could end her life in this tragic way. We spent many wonderful years together and had made a great life for ourselves.”

Ms. Scott was media averse, preferring not to exploit her friendships with celebrities. “I just want to be known for what I do,” she told New York magazine in 2008, “not who I know.”

She was raised by Mormon adoptive parents in Roy, Utah. With raven locks and an Amazonian figure — she was 6 feet tall at age 12 — she stood out.

She was a self-taught seamstress, tailoring thrift-store finds to fit her frame. “I was tall, so I couldn’t find ready-to-wear clothes that were long enough,” she told the Scotsman newspaper. “I would buy old vintage dresses in beautiful silk moiré and taffeta, and recut them to suit my proportions.”

As a teen, fashion photographer Bruce Weber spotted her in Utah during an advertising shoot and encouraged her to pursue a modeling career. At 17 and then known as Luann Bambrough, she used her babysitting earnings to buy a one-way ticket to Paris, where she reinvented herself as L’Wren Scott.

“Naïveté and fearlessness are two great things you have when you’re young,” she told Women’s Wear Daily in 2010.

A print ad for Pretty Polly legwear — which displayed her strikingly long legs — led to jobs in Chanel and Thierry Mugler couture shows and doing photo shoots with fashion designer Karl Lagerfeld.

She moved to Los Angeles in 1994 to do publicity for Prada and helped orchestrate the successful advertising campaign for Elizabeth Taylor’s best-selling fragrance White Diamonds. Shortly thereafter, she began her career behind the camera as a stylist working with American fashion photographer Herb Ritts.

She soon segued into costuming work for films, outfitting fashion-conscious stars such as Sharon Stone in “Diabolique,” Kidman in “Eyes Wide Shut” and Ellen Barkin in “Mercy” and “Oceans Thirteen.”

Her marriage to London-based property developer Anthony Brand ended in divorce. A complete list of survivors could not be confirmed.

“When someone feels confident, she shines,” Ms. Scott told the Scotsman. “When I do my job as a designer, I focus on designing clothing so my customer feels fabulous in a dress and feels confident going out and owning it.”