Hugh James Leonard, 51, the telephone service manager of the Facilities Service Department of Montgomery County, died Tuesday in a hospital in Orangeburg, S.C., after a heart attack.

He was stricken while he and his wife, the former Mary Ann Schuster, were driving back to their home in Lanham after attending the funeral of Mrs. Leonard's father in Florida.

After serving in the Army from 1951 to 1953, Mr. Leonard was a telephone service manager with the 3M Corp. in his native St. Paul, Minn., for 18 years before moving to this area in 1970. He was an office manager for the Macke Co. and the accounting firm of Ernst & Ernst and was an administrative assistant at the Shrine of the Immaculate Conception before joining the Montgomery County government in February 1977.

He was a member of Saint Matthias Catholic Church in Lanham.

In addition to his wife, survivors include two sons, Timothy, of Lanham, and Joseph, of St. Paul; a daughter, Therese, of Lanham; three sisters, Leona Engert of Kansas City, Mo., Madonna Leonard of St. Louis Park, Minn., and Sister Therese, O.S.U., of Alton, Ill.; and a brother, Harold, of South St. Paul, Minn.