For Tadashi Yanai, the founder and president of the Uniqlo parent company Fast Retailing, the United States has become a source of both immense optimism and concern.

In recent years, Uniqlo’s affordable, well-made basic clothes have struck a chord, especially with younger American shoppers, fueling a rapid business expansion. The United States is now a pillar of the growth strategy of Japan’s Fast Retailing.

Mr. Yanai, 76, came of age in postwar Japan, steeped in American culture. He wore Converse and drew inspiration for Uniqlo from casual wear brands like Gap. Conquering the American market is also a matter of personal legacy for Mr. Yanai, whose Uniqlo empire today comprises more than 2,500 stores globally.

“I want to succeed in America,” Mr. Yanai said in an interview in a sunlit office in New York’s Meatpacking district. It was the preppy youth styles of the East Coast that opened his eyes to clothing in the 1960s and 1970s, Mr. Yanai said. So, he said of the market, “I feel the most attachment to it.”

After opening its first store in New Jersey in 2005, Uniqlo’s North American business didn’t turn profitable until 2022. The market’s expansion since then helped Fast Retailing report a record annual net profit of $2.8 billion on Thursday.

In the United States, “finally, we got to the entrance point,” Mr. Yanai said.

Yet Mr. Yanai’s optimism is shadowed by his growing alarm over the political and economic direction of the country that played a foundational role in Uniqlo’s identity.