Kate Sylvester grew up in Greenhithe, on Auckland's North Shore, and was taught to sew by her mother, Toni. She studied textile and clothing design at Wellington Polytechnic in 1985 and 1986 and began her career hand-sewing screen-printed T-shirts. In London she worked at Liberty and as a machinist for Arabella Pollen, and in Paris for Corinne Robson.

Back in New Zealand she founded her first label, Sister, with her partner Wayne Conway, advertising it with street graffiti; the first Sister store opened on Kitchener Street, Auckland, in 1993. In 1997, after the name met resistance in Australia from a label called Sista, they renamed the business Kate Sylvester. Her first show under the new name, the Arts and Crafts collection in Sydney in 1999, set off a contest between Barneys and Henri Bendel in New York.

Sylvester's signature combines sportswear, lingerie and traditional tailoring, filtered through the fashion of the 1930s and 1950s. She is a literary designer: collections have taken their titles from Catcher in the Rye, Love in a Cold Climate and Brighton Rock, and This Charming Man drew on Proust. Her eyewear has honoured Sylvia Plath, Harper Lee, Janet Frame and Eleanor Catton, and Art Groupie referenced the Surrealists and Klimt's The Kiss. Over time she added a diffusion line, eyewear, accessories, lingerie and homeware.

When Jacinda Ardern was sworn in as prime minister in 2017 she wore Kate Sylvester's blue and red Nadia dress with a single-breasted jacket; Ardern later gave the outfit to Te Papa. Sylvester also co-founded Mindful Fashion New Zealand with Emily Miller-Sharma of Ruby, a body promoting a circular and sustainable local industry. Her partner Wayne Conway, a graphic designer, created the label's prints and ran the business side. "There's no way I could do it on my own," she has said. "He's as much the brand as I am." The business, the couple said, never ran at a loss.

In April 2024 the couple announced that the label would close after 31 years. "We want to go out the way we came into this business, still loving what we do," they said; Kate Sylvester sold its final dress in March 2025. The former Fashion Quarterly editor Fiona Hawtin remarked that "everyone wore Kate at one stage", and that her shows were more than parades of clothes: "it was an emotional event". The couple chose to close rather than sell the name.

Their three sons, Ike, Tom and Cosmo Conway, all Massey University design graduates, relaunched Sylvester, the more casual diffusion line their mother had introduced in 2004, in April 2025, with the label's long-time designer Christine Leung alongside them and new stores opening that month. In July 2026, citing New Zealand's economic climate, the family announced that Sylvester too would wind down by February 2027, with that summer's collection its last.

Her honours include induction into the Massey University College of Creative Arts Hall of Fame (2008), an honorary doctorate from Massey (2011) and appointment as an Officer of the New Zealand Order of Merit in the 2025 New Year Honours.