Lawrence A. Pezzullo, an American diplomat who in 1979 negotiated the abdication of Anastasio Somoza DeBayle as leader of Nicaragua and the demise of the dictatorial dynasty that had led the country with Washington’s sponsorship for four decades, died on Wednesday in Baltimore. He was 91.

The cause was heart failure, his son Ralph said.

Mr. Pezzullo also tried, though less successfully, to negotiate a return to civilian rule in Haiti in the 1990s, and he ran the international Catholic Relief Services for a decade. But he is best remembered as a voice of the Carter administration’s commitment to human rights, an effort that culminated in Somoza’s departure.

As the incoming United States ambassador, Mr. Pezzullo was sent to Nicaragua with a challenging mission: to persuade Somoza to avoid further bloodshed in his country’s protracted civil war with the leftist Sandinistas by relinquishing power to them and fleeing into exile, initially in Miami.

Somoza, whose family had led Nicaragua since the mid-1930s, was first elected president in 1967 and gained a reputation for widespread corruption and brutal repression of dissenters. But he was supported by the United States because he was fervently anti-Communist.