But as a public corporation, Ben & Jerry’s had a fiduciary duty to its shareholders. After much hand-wringing, it agreed to a deal with Unilever for $326 million. And despite handsome paydays — Mr. Cohen’s stake was reportedly worth about $40 million, while Mr. Greenfield’s was worth about $10 million — the founders said in a statement that they would have preferred to remain independent.

Very quickly, some of their worst fears were realized.

Ben & Jerry’s had almost never fired an employee, but Unilever, the world’s largest ice cream maker, had manufacturing and supply-chain acumen and wanted synergies. A production plant and a distribution center were shuttered, workers at the two sites were laid off, and sales representatives at headquarters were fired.

“There were redundancies and consolidation,” said Rob Michalak, who has worked for Ben & Jerry’s on and off for 26 years and is now director of social mission. (He met Mr. Greenfield while hitchhiking in 1978.) “That created unease and suspicion.”

Then, shortly after the deal, in an affront to the company’s political sensibilities, Unilever reportedly prevented employees from emblazoning the Ben & Jerry’s logo on a bus driving them to a protest.

Kevin Havelock, president of refreshments at Unilever, likened the takeover to a new marriage. “There was a lot of learning that had to take place,” he said. “Where there have been challenges, we’ve talked them through.”

But today, 15 years after the deal, Mr. Michalak said Ben & Jerry’s remained as mission-driven as ever, and was having a bigger impact than before because of its increased size. (Since the acquisition, Unilever has nearly tripled Ben & Jerry’s revenue and added hundreds of jobs.) And instead of watching Ben & Jerry’s simply disappear into Unilever, Mr. Michalak and his colleagues have pushed their new parent company to become a more progressive multinational.

The recipe for this amicable partnership was written into the acquisition agreement. Unilever, not wanting to squander its purchase, chose to operate Ben & Jerry’s with more autonomy than any of its other subsidiaries. To ensure middle managers did not contaminate the unique culture, Unilever established an “external board” charged with overseeing Ben & Jerry’s culture and social mission.