He was named ambassador to Iran in 1977 by President Jimmy Carter at a time that restive Iranians were growing tired of the rule of Shah Mohammed Reza Pahlavi. In 1979, Sullivan was instructed to tell the shah that the United States felt he should leave Iran. Ayatollah Khomeini, the exiled revolutionary leader, returned to Iran in early 1979, and revolutionary forces toppled the monarchy.
Several days later, heavily armed guerrillas attacked the U.S. Embassy. Sullivan directed the defense of the embassy and ordered Marines to use tear gas to keep the attackers out while staff destroyed files and equipment. The attackers captured Sullivan and about 100 other Americans and they were briefly held hostage, but pro-Khomeini forces were able to gain control of the embassy from the attackers.
Sullivan left Iran soon afterward and retired from the Foreign Service later in 1979. Sullivan's experience as a hostage in Iran came nine months before the seizure of hostages at the U.S. Embassy that lasted from November 1979 until the end of the Carter presidency on Jan. 20, 1981.
Sullivan was born in Cranston, Rhode Island, in 1922. He graduated from Brown University and served in the Navy during World War II.