Kenneth Chenault, the longtime CEO and Chairman of American Express AXP

The retirement of Chenault, who has been CEO since 2001, will leave just one black executive at the helm of a Dow 30 company, according to the Executive Leadership Council, an organization that works to boost diversity in corporate America. That leader is Kenneth Frazier, chairman and CEO of Merck.

American Express said on Wednesday that Chenault, 66, will be replaced by Stephen J. Squeri, 58, on February 1 of next year.

Related: Beth Comstock, first female vice chair at GE, out in executive shakeup

The number of black CEOs has dropped over the past few years. Ursula Burns left her post as head of Xerox last year when the company split in two, and Rodney O'Neal retired from his role as CEO and president of Delphi in 2015. Don Thompson retired as CEO of McDonald's in 2015, as well.

"We're starting a new chapter from a position of strength and this is the right time to make the leadership transition," Chenault said in a statement. He joined American Express in 1981.

Chenault was praised by Warren Buffett, chairman and CEO of Berkshire Hathaway, who said "Ken's been the gold standard for corporate leadership and the benchmark that I measure others against."

"No one does a better job when it really counts and he's always done it with the highest degree of integrity," Buffett added.

But not everyone has been pleased with the leadership at American Express. Shareholder Trip Miller with Gullane Capital Partners said "we've been disappointed in management for a long time." Of Squeri, who has been vice chairman since 2015, Miller added, "we're not overly thrilled with this selection." Squeri joined the company in 1985.

Miller had hoped that AmEx would tap an outsider to help the financial services firm lose its "older, stodgier" reputation and become a stronger competitor to Millennial-focused companies.

Still, Miller feels good about AmEx. "American Express is still a great brand, regardless of who is running it or has been running it," he said. "It's stood the test of time."