Donald A. Mahley, a retired Army colonel and nuclear weapons officer who went on to become the State Department’s chief nonproliferation negotiator with the rank of ambassador, died March 1 at his home in Vienna, Va. He was 71.

The cause of death was pancreatic cancer, said his wife, Julianna Mahley.

Amb. Mahley was a State Department special negotiator of nonproliferation matters for several years before being appointed deputy assistant secretary for arms control in 2002. Two years later, he was appointed the deputy assistant secretary for threat reduction, export controls and negotiations. He retired in 2008, but remained a part-time negotiator on an as-needed basis, most recently acting as the U.S. representative of the U.N. Arms Trade Treaty discussions.

From 1995 to 2001, Amb. Mahley led negotiations for an ultimately unsuccessful protocol that aimed to strengthen the 1972 Biological Weapons Convention, an international treaty that prohibited germ warfare. He also oversaw the U.S. effort to dismantle Libya’s nuclear and chemical programs in the early 2000s.

Earlier in his career, he was the managing director of the U.S. National Authority for implementation of the Chemical Weapons Convention and the deputy assistant director of multi­lateral affairs at the U.S. Arms Control and Disarmament Agency.

Donald Arthur Mahley was born in Peru, Ind. He was a 1964 philosophy graduate of Purdue University in Indiana, where he joined the ROTC and was a nationally ranked debater. He received a master’s in public and international affairs from the University of Pittsburgh in 1971.

He served 27 years in the Army before retiring in 1992. Early in his military career, he commanded two ammunition depots in Vietnam and ran a nuclear missile defense unit in Turkey.

Later, he served as a deputy director at the U.S. mission to NATO in Brussels and was a director of defense policy and arms control at the National Security Council.

He was a U.S. representative to the Organization for the Prohibition of Chemical Weapons and served on the U.N. secretary general’s advisory board on disarmament matters.

He was on the board of directors of Synetic Theater in Arlington.

Survivors include his wife of 45 years, Julianna Stephenson Mahley of Vienna.

— Megan McDonough