A dispute over creative control led Ralph Lauren Corp. Chief Executive Stefan Larsson to leave the struggling luxury fashion brand after less than two years at the helm.

Mr. Larsson, a 42-year-old fast-fashion executive tapped to work alongside founder Ralph Lauren, said Thursday that the two men agreed the business needed to evolve but disagreed over the company’s creative and customer-facing strategies.

“The board, Ralph and I have over the last month worked very hard to find common ground,” Mr. Larsson said on a conference call with investors, calling his abrupt exit a mutual decision. “However, we have found that we have different views on how to evolve.”

Shares fell 12% in New York Thursday, and are now down more than 30% over the past year. Ralph Lauren also reported another slide in quarterly sales.

Executives said turnaround efforts led by Mr. Larsson—which included closing stores, pulling back on discounts and jettisoning some of the company’s smaller labels—were on track, and promised smaller revenue declines next fiscal year.