Bill Ackman is known among investors for making bold calls that nobody believes in. But the billionaire activist investor and founder of Pershing Square Holdings, which had $16.1 billion of assets under management through March, said his decision to invest in the British watch company Bremont was not one of them.

“I was at a conference where Jeff Bezos spoke, and he said that in life it’s really important you have some whimsy,” Mr. Ackman said in a call last month.

He found that whimsy in the summer of 2022 when he was in Britain to attend his daughter’s graduation from Cambridge and visited a Bremont boutique in the upscale London district of Mayfair, where he bought eight watches. Shortly after, he got in touch with the company’s founders, the brothers Nick and Giles English, and by that October had taken a small stake in the business. “I started out as a very passive investor in the company,” he said.

His passivity didn’t last. In January 2023, it was announced that Mr. Ackman and a longtime Bremont investor, the New York-based Hellcat Acquisitions, had taken a substantial minority stake in the company in a funding round worth 48.4 million pounds (then $59 million), at the time giving it a market valuation of over £100 million. Mr. Ackman, who Forbes recently estimated to have a net worth of close to $9 billion, had invested privately.

It prompted a period of significant change for the watchmaker. By the end of that year, the English brothers had stepped away from the day-to-day running of the business, and, under Mr. Ackman’s recommendation, the watch industry veteran Davide Cerrato had been installed as chief executive.