John L. Hamilton, 68, a partner in the law firm of Hamilton & Hamilton, the oldest in Washington, died of cancer Saturday at Georgetown University Hospital.

Mr. Hamilton joined the firm in 1938, the year he earned his law degree from Georgetown University. In the course of his career there, the firm was general counsel to the Archdiocese of Washington, division counsel of the Southern Railway and counsel to The Washington Star Newspaper Company. Mr. Hamilton himself was the general counsel of the Washington Board of Trade as well as a member of its board of directors. He also was a member of the board of the McLachlen National Bank.

He was a past president of the Barristers and a member of the Lawyers Club.

Apart from his law practice, Mr. Hamilton took part in numerous civic, school and church activities. From 1951 to 1980, he was president and chairman of the board of the Holton Arms School in Bethesda. He also was a trustee of the Landon School in Bethesda and the Canterbury School in New Milford, Conn.

He was a trustee of the St. John's Child Development Center and a member of the board of managers of the Hospital for Sick Children.

A descendent of a family that settled in Maryland in 1670, Mr. Hamilton was born in Washington and lived in the city for the rest of his life. He attended Sidwell Friends School here and graduated from the Canterbury School in New Milford. He graduated from Princeton University before studying law at Georgetown.

During World War II, he served in the Navy. He was an intelligence officer in Washington and then the gunnery officer of an escort carrier in the Pacific. He took part in the campaigns in the Mariana Islands, the Philippines and Okinawa. He retired from the reserves with the rank of lieutenant commander.

He was a past commander of the Augustus P. Gardner Post of the American Legion.

Mr. Hamilton also was a member of the Chevy Chase Club, where he was on the board of governors for many years, the Metropolitan Club, the Princess Anne Country Club in Virginia Beach, Va., and the Colonial Club at Princeton University. He was a sponsor of Ducks Unlimited.

Survivors include his wife, the former Patricia Eleanor Paine, of Washington; two sons, Dr. Colin Whiting Hamilton, of Virginia Beach, and John Craig Hamilton, of Washington; one sister, Mrs. Victor R. Alfaro, also of Washington, and two grandsons.