Arnold J. Sagalyn was born in March 2, 1918 in Springfield, Massachusetts. He graduated from Oberlin College in Ohio and the Graduate Institute of International and Development Studies in Geneva, Switzerland.
In 1939, he became a special assistant for Eliot Ness, who at the time was the safety director of Cleveland, Ohio. He helped Ness in reorganization of the city's police department. In 1942, when Sagalyn worked for the Cleveland Press, he was sent to investigate a suspect in notorious series of murders, known as the Cleveland Torso Murders case. The suspect from Maysville, Kentucky, known as the "Kentucky butcher", had committed a grisly murder and, enjoying ties with the local government, had received a brief sentence. Sagalyn failed to interview him. He later told this story to James Jessen Badal during making of his book about the Cleveland murders. The same year, Sagalyn came to Washington where he helped to organize a nationwide law enforcement program against prostitution.
In the years of military service during the World War II Sagalyn was an aide to the Chief of the Public Safety Division of the Office of Military Government for Germany. There he helped reorganizing the German police system.
As a journalist, Sagalyn worked for Life magazine in 1947–1949 and for The New York Times in 1949–1952. He worked as writer and producer for NBC between 1952 and 1954. He was a partner in Mountain Fir Lumber Company next three years and an assistant publisher for Northern Virginia Sun since 1957.
In 1961, Sagalyn was named director of the United States Department of the Treasury's Office of Law Enforcement Coordination, where he advised the Secretary on law enforcement policy and coordinated the operations of the Treasury Department’s enforcement agencies. These included the U.S. Secret Service; the Bureau of Narcotics; the Bureau of Customs’ Investigations and Enforcement Division; the Internal Revenue Service’s Intelligence, Inspection and Alcohol and Tobacco Tax Divisions; and the U.S. Coast Guard’s Intelligence Division.
In 1965 he became the Treasury Representative to the President's Commission on Law Enforcement and Administration of Justice. At the same time he served as the Advisor on Public Safety to the Secretary of Housing and Urban Development. In 1967 he served as an associate public safety director for the National Advisory Commission on Civil Disorders.
While at the Treasury Department, Mr. Sagalyn served as the US representative for the International Criminal Police Organization (INTERPOL) from 1961 to 1967 and was Senior Vice President from 1962 to 1965.
In September 1968 Sagalyn left government service, switching to private consulting, working for Arthur D. Little, Inc. In 1970 he founded Security Planning Corporation, a research and consulting firm, specialized in crime prevention and security and risk management. The company was a partnership with his spouse Louise Sagalyn, Peter Labovitz and John Labovitz. By the 1990s he ran the consulting firm Sagalyn Advocates.
Sagalyn donated a collection of his papers and books to the American University library in March 2004 and again in July 2012. These papers include his works of 1931–2004, covering primarily the topics of law enforcement, police work, terrorism, and civil disorder, and a memoir published in 2010.
Arnold Sagalyn married Louise London (born 1927), a lawyer, in 1957. He had three sisters, Esther S. Bick (1914–2012), Lenore Cohen and Lillian Burack, and three brothers, Julian, Robert and Irwin Sagalyn; all of them deceased. He died at the age of 99 at his home in Washington on September 11, 2017.