Jenna Lyons, the woman who gave American preppy retailer J Crew the cool factor, has exited the company after 26 years.

Whether or not you know it, Lyons has had an influence on your wardrobe – particularly in the last two decades. Having joined J Crew in 1990, it was chief executive Mickey Drexler who promoted her when he came to the company in 2003.

Lyons pinpointed a unique position for J Crew in the American market. Long the go-to store for preppy staples such as button-down shirts and cashmere sweaters, the Lyons effect added bright colours, precious fabrics and statement jewellery. In the designer’s eyes, clashing prints and ideas worked. See stripes worn with camouflage and precious sequins contrasted with denim. Bold, beaded necklaces, sequined ballet flats and oversized blazers became bestsellers. Sequins, reserved for special occasions, became a neutral in Lyons’ world.

Camo, tartan and tulle … the Lyons effect. Photograph: John Lamparski/WireImage

Lyons was an ambassador for the label as well as a designer. A statuesque woman, she became well-known for her signature black-framed glasses, coral lipstick, trousers and stilettos. The half-tuck – tucking your shirt in at the front, but not the back – is a long-term Lyons trick. Shrobing – or shoulder robing (wearing your coat around your shoulder), as seen on Theresa May in Vogue – is a key Lyons styling trick. An appearance at the Met Ball in 2011, where she teamed a ballgown with a sweatshirt, was hailed on influential blog Man Repeller as “both revolutionary and obvious, the type of pairing that makes you wonder why you didn’t think of it first”.

Michelle Obama on The Tonight Show with Jay Leno in 2008. Photograph: NBC via Getty Images

J Crew’s other style icon was Michelle Obama. The first lady frequently wore the brand during her eight years in the White House – many of her famous cardigans came from J Crew. Obama’s penchant for the label – so associated with the American preppy look – was first flagged up during the election campaign in 2008. Appearing on Jay Leno in a yellow J Crew skirt suit, this relatively affordable choice was contrasted with the news that the Republican national convention had spent $150,000 (£120,000) on high-end clothes for Sarah Palin when she was chosen as the Republican vice-presidential candidate. It might be a stretch to say that J Crew put the Obamas in the White House but the label certainly had a hand in creating the off-duty Obama brand: fun, approachable and all-American.

Such endorsements also helped J Crew grow financially. In 2010, the Harvard Business Review calculated that Michelle Obama wearing a brand’s clothes could lead to sales of $14m (£11.25m), and the brand frequently had to restock the cardigans and other items that the first lady wore.

This was during the heyday of J Crew. In the third quarter of 2009, sales rose 14% and by 2013, the year J Crew opened in London’s Regent Street, revenue reached $2.2bn (£1.76bn). However, in 2016, sales were down 6% on the previous year. The decline has been attributed to the growth of very cheap, fast fashion.

J Crew SS17 at New York fashion week in 2016, with Jenna Lyons in the front row. Photograph: Sam Deitch/BFA/Rex/Shutterstock

In a statement, Drexler was typically relaxed about Lyons’ exit. “Jenna and I got together and we both agreed it was time for a change,” he told Business of Fashion. “That being said, she’s got plans to do other things. It’s been a great run. There’s a lot of mutual respect between Jenna and me.” Perhaps because of this relationship, Lyons will stay on at J Crew as a creative adviser until the end of the year.

Lyons’ plans for 2018 will be a matter of close scrutiny for the fashion industry, particularly in the US. Her own statement gives little away. “I am excited about the next chapter for J Crew as well as the opportunity for other creative leaders within the organisation to step up and take on new responsibilities,” she says. “Having spent the better part of my life with J Crew, I feel an immense pride and love for everyone at the company.”

Somsack Sikhounmuong, who has worked with Lyons at J Crew since 2001, will become the chief design officer, suggesting that the safe pair of hands that comes with a long-term team member will continue to be highly prized at the company. “Somsack and our design teams have a deep understanding of the aesthetic and style our customers rely on us to deliver, with a proven track record of driving creative vision in line with our brand DNA,” says Drexler. Sikhounmuong cut his teeth on Madewell, the sister company to J Crew, designed for the millennial market. Sales at Madewell rose 14% in 2016, to $341.6m (£273.85m).