After 30 years of leading the Museum of Modern Art, during which he moved one of the country’s leading museums from the 20th century into the 21st, Glenn D. Lowry announced on Tuesday that he would step down as its director in September 2025.

“It’s the right moment to think about the future of the museum and I just thought, carpe diem,” Lowry said in an interview. “All the things I set out to do 30 years ago are either accomplished or in play in a very positive way.”

Lowry’s contract was up for renewal in June. He said he felt invited to continue but decided it was time to turn over the reins to the next generation of leadership.

“I didn’t want to be the person who stayed too long,” he said.

The board will start an international search for his successor, the New York museum said. Marie-Josée Kravis, the museum’s chairwoman, said that Lowry’s decision to move on was “by mutual agreement” and that his contract “could have been renewed.”

Replacing Lowry — who turns 70 this month — will not be easy, given his long, successful tenure and because the job description of museum director has grown increasingly complicated in recent years amid economic straits, the coronavirus pandemic and social unrest.

Presiding over a major museum these days requires considerably more than a Ph.D. in art history and the ability to hire capable curators. Directors must be culturally sensitive diplomats — able to communicate effectively with artists, trustees and potential activists — as well as expert fund-raisers, with cultural institutions depending more than ever on private donations.