And then there were three.

When Kenneth Chenault, chief executive officer of American Express Co., steps down Thursday, the number of African-American CEOs in the Fortune 500 will drop to three, a 50 percent decline in the past two years. Franklin Raines first broke the color barrier in 1999; Chenault became the fourth when he took the reins at the credit card giant in 2001.

Kenneth I. Chenault
Kenneth I. Chenault Bloomberg News

“I was encouraged the first few years that I was CEO because we saw an emergence" of more minority CEOs, Chenault said in an interview earlier this week with the Associated Press. He is being replaced by Steve Squeri, who is white. “You started to see some movement. And that has slowed down.”

The remaining black CEOs include Roger Ferguson at TIAA, Kenneth Frazier at Merck & Co. and Marvin Ellison at J.C. Penney Co. Of the 15 African-Americans who have served as CEO since 1999, there has been only one woman among the group, Ursula Burns at Xerox Corp.

Social movements in the 1960s and 1970s laid the groundwork for that small fraternity of African-Americans to lead big companies, said corporate historian Richard Zweigenhaft. “When there’s a barrier like that that has not been broken, there’s a lot of pressure on the people who are making those kinds of decisions to be open to change.”

But once that wall falls, he added, “one of the ironies is that there’s less pressure to change.”

Bloomberg News