Lionel A. Rosenblatt, a U.S. Foreign Service officer who helped roughly 200 South Vietnamese citizens evacuate Saigon days before it fell in 1975 with a daring and unauthorized mission that prefaced a career advocating for refugees in Southeast Asia and other global hot spots, died on April 11 at his home in Washington, D.C. He was 82.

The cause was leukemia, his wife, Ann Rosenblatt, said.

On April 20, 1975, Mr. Rosenblatt, then 31, flew out of Washington bound for Saigon, the South Vietnamese capital, which would be captured by the North Vietnamese Army and the Viet Cong on April 30 and renamed Ho Chi Minh City.

Traveling with him was a colleague, Larry Craig Johnstone. They belonged to a small group of Foreign Service officers who met daily over lunch in Washington, concerned about the fate of the Vietnamese who had worked with the United States during the war, tens of thousands of whom could be vulnerable to retribution by the communist North Vietnamese regime.

When Graham Martin, the U.S. ambassador to South Vietnam, suggested that most Vietnamese refugees should head to coastal points, where American ships would attempt to pick them up, Mr. Rosenblatt and Mr. Johnstone, both of whom had served in diplomatic postings in Vietnam, grew alarmed at his seeming indifference toward the Vietnamese who were not high-ranking officials. So they secretly devised their own evacuation plan to assist former colleagues and their families.

“These were people we had worked with hand in glove,” Mr. Johnstone said in an interview. “Our lives depended on their ability to help us. To up and leave them was unthinkable.”