William C. Mooney, 43 a Senate committe aide for 10 years and then an employee of the Law Enforcement Assistance Administration, died at his home in Clinton, Md., Sunday following an apparent heart attack.

Mr. Mooney was on a special assignment to the White House at the time of his death to work on President Carter's juvenile justice reorganization plan.

He began his government career in 1962, when he joined the staff of the Senate Juvenile Delinquency Subcomcommitte. He remained there until 1972 and then moved over to LEAA. It was the LEAA that sent him to the White House to help with the juvenile justice project.

Mr. Mooney was born in Lee, Mass., and grew up in Winstead, Conn. He graduated from the University of Connecticut. He served three years as an intelligence officer in the Army. Latter, he moved to the Washington area and earned a master's degree in criminal law at George Washington University.

Survivors include his wife, Kay, and three children, William charles III, Mary Kathleen, and Elizabeth Ann, all of the home in Clinton; his mother, Mary V. Mooney, of Winstead, Conn; a brother, Edward, of Washington, and a sister, Elizabeth Brown, of Kensington, Conn.

The family suggest that expressions of sympathy be in the form of contributions to the Clinton, Md., Volunteer Fire Department Paramedic Program.