Robert L. Thornton, 70, a retired civilian official of the Army Corps of Engineers, died of cancer Saturday at Fairfax Hospital. He was a resident of Reston.

Mr. Thornton, a civil engineer, worked for the Corps of Engineers for 30 years. When he retired in 1969, he was acting deputy chief of its Systems Engineering Laboratory at Fort Belvoir.

He was a 1969 recipient of the Army's Meritorious Civilian Service Award and its Certificate of Achievement.

A native of New York City, Mr. Thornton moved to Washington in the 1930s. He began his government career as a surveyor with the U.S. Coast and Geodetic Survey in 1934 and joined the Corps of Engineers five years later.

He was a graduate of the Army Management School at Fort Belvoir.

Mr. Thornton lived in Long Boat Key, Fla., for 10 years before returning to this area in 1979.

Survivors include his wife, Dorothy Dixon Thornton of Reston; a daughter Dora Anne Hedlund of Scarsdale, N.Y.; a son, William L., of Wilmington, Del.; three stepsons, Navy Lt. Cmdr. Lewis R. Dixon of Newport, R.I, Navy Lt. James R. Dixon of Boston, and Richard A. Dixon of Cape Canaveral, Fla., and nine grandchildren.