Paul DeWitt Mason, 49, chief of the investigation division of the U.S. Information Agency, died of cancer Tuesday, at the Washington Hospital Center.

He joined USIA in 1962 as deputy chief of its special investigations branch. He left for several months in 1968 to take an assignments in New York City with the Bureau of Narcotics and USIA as head of the investigations division in the office of security.

Mr. Mason had been active in federal security matters since 1955, when he became an investigator for the Central Intelligence Agency. He later worked in similar positions with the Internal Revenue Service and State Department before joing USIA.

Born in Detroit, he grew up in Skyland, N.C. After graduating from high school there, he started work with the federal government in the General Accounting Office's postal accounts division in Ashville, N.C.

After serving in the U.S. Army, when he saw duty in Japan, Mr. Mason worked in his way through the University of North Carolina, where he received a bachelor's degree in personnel relations in 1954.

In 1970, he was elected to the National Executive Committee of the Association of Federal Investigators.

He is survived by his wife, Jacqueline Mason, and four children, Reed, Michael Jacqueline and Heather, of the home in Rockville.

The family suggests that expressions of sympathy may be in the form of contributions to the University of North Carolina at Chapel Hill.