Spurgeon M. Keeny Jr., an arms control expert who held top positions at the U.S. Arms Control and Disarmament Agency during the Cold War and later ran an influential Washington think tank that advises policymakers on nuclear proliferation, died Aug. 10 at his home in Washington. He was 87.

He had cancer, said his daughter, Virginia Keeny.

Mr. Keeny was a prominent scholar in his field at a time when the U.S.-Soviet arms race was one of the most important national concerns. He was known as a meticulous strategist with a pragmatic worldview: that the nuclear threat was best managed through the incremental drawdown of arms.

A Soviet expert, Mr. Keeny gained much of his early experience as an assistant to the White House science adviser during the Eisenhower, Kennedy and Johnson administrations. He also served on the staff of the National Security Council from 1963 until 1969, when he moved to the Arms Control and Disarmament Agency as assistant director for science and technology. In the mid-1970s, he led an influential study on nuclear power underwritten by the Ford Foundation.

Mr. Keeny held his most prominent position at the agency during the Carter administration. In 1977, he became deputy director, a post that often put him in charge of day-to-day operations while the director was traveling or at work on negotiations. He was reported to have been handpicked by the director, Paul C. Warnke.

Spurgeon M. Keeny Jr., who held the No. 2 position at the U.S. Arms Control and Disarmament Agency during the Carter administration and later ran an influential Washington think tank that seeks to help curb nuclear proliferation, died Aug.10. (Washington Post File)

At the time, the United States and the Soviet Union were engaged in the second round of strategic arms limitation talks, known as SALT II. Mr. Keeny “was involved with every interagency decision with our negotiations with Moscow at this time,” said Michael Krepon, a nuclear expert who worked at the agency during the Carter administration.

After the Soviet Union invaded Afghanistan in 1979, support for the SALT II treaty evaporated. The agreement was significant nonetheless because it would have been the first to limit the number of nuclear warheads — as opposed to missiles — that the superpowers could maintain.

In 1981, after Carter was defeated for reelection, Mr. Keeny joined the National Academy of Sciences. In 1985, he became president of the Arms Control Association, which was formed in 1971. He retired in 2001.

Under Mr. Keeny’s leadership, Krepon said, the association was “the go-to place for all those looking for an alternative to a nuclear arms race.” Mr. Keeny testified before Congress and was frequently quoted in the media. The association’s flagship publication, Arms Control Today, remains an influential resource for policymakers and others.

Spurgeon Milton Keeny Jr. was born Oct. 24, 1924, in New York City. His father became a noted United Nations and Population Council expert on family planning in Asia.

Mr. Keeny received a bachelor’s degree in 1944 and a master’s degree in physics in 1946, both from Columbia University, where he also did graduate work in Soviet studies.

From 1948 until 1955, he did intelligence work with the Air Force, including two years of active duty. He rose to become head of the office that tracked the Soviet nuclear program.

Mr. Keeny’s professional honors included the Rockefeller Public Service Award, given to him by the federal government in 1970. From 1973 to 1977, he worked at the Mitre Corp., a nonprofit technology firm that works in defense and other fields.

His wife of 59 years, Sheila Spear Keeny, died in 2011. Survivors include three children, Christopher Keeny of Edgemont, N.Y., Virginia Keeny of Los Angeles and Spurgeon M. Keeny III of Chevy Chase; and four grandchildren.

Krepon said in an interview that Mr. Keeny enjoyed the artwork of Hieronymus Bosch, the Dutch painter whose 15th- and 16th-century works often feature mystical creatures such as gremlins.

Mr. Keeny spent his career “working on rather large gremlins,” Krepon said, “so there might have been a connection.”