Hugh Caldwell Clagett Sr., 63, president and treasurer of the architectural firm of Mills, Clagett, and Wening, died of cancer Saturday at his home in Upper Marlboro.

He was a founding partner of the firm in 1975. It has offices in Washington, Upper Marlboro and Chevy Chase. Earlier Mr. Clagett had been with the Washington architectural firm of Mills, Petticord, and Mills for 23 years.

He was associated in the designing of buildings at George Washington University, the Smithsonian's Museum of History and Technology and the National Geographic Society building near Darnestown, Md.

Mr. Clagett was born in Washington. He was a graduate of old Central High School. He served as a lieutenant with the Navy Seabees in World War II and participated in the Normandy invasion.

He earned a degree in architectural engineering from Catholic University after the war.

Mr. Clagett helped in the planning for a proposed National Tobacco Museum in Upper Marlboro.

He was a founder of Queen Anne School in Upper Marlboro.

He belonged to the Marlborough Hunt Club, the Southern Maryland Society, the American Society of Civil Engineers, the National Society of Professional Engineers, the Washington Building Congress, the Metropolitan Washington Board of Trade and the Maryland Historical Trust.

He is survived by his wife, the former Katherine Berry, and four children, Kathleen, Margaret, Hugh C. Jr., and Elizabeth, all of Upper Marlboro, another daughter, Alice, of San Diego, Calif., and five brothers, John D., George G. and Frank M., all of Upper Marlboro, David C. Sr., of Columbia, Md., and Charles T. Sr., off Edinburg, Tex.

The family suggests that expressions of sympathy be in the form of contributions to the Discalced Carmelite Sisters in LaPlata, Md.