John Hamilton Walker, 68, a retired executive vice president of the Society of American Florists in Alexandria and a former assistant supervising agent of the White House Secret Service detail, died of a heart attack Wednesday at his home at Hilton Head Island, S.C.

Mr. Walker, who joined the society in 1956, was instrumental in establishing its endowment fund, as well as the American Florists Marketing Council. He became a certified association executive in 1962. In 1978, when he retired and moved to South Carolina, he was elected to the Horticultural Hall of Fame. He also received the Key Award of the American Society of Association Executives, its highest award.

He was a past president of the American Horticultural Society and served on the boards of the National Council of Therapy and Rehabilitation Through Horticulture, the National Junior Horticultural Association and the Association of Former Secret Service Agents.

Before beginning his career in association management, he was an officer of the National Geographic Society for six years.

From 1943-48, Mr. Walker was with the U.S. Secret Service, where he was an authority on presidential protection. Besides serving as assistant supervising agent of the White House detail, he was assistant to the director of the Service.

He was born in Braddock, Pa., graduated from Slippery Rock State College in Pennsylvania and did postgraduate work at the University of Pittsburgh.

He served in the Army overseas in World War II.

Mr. Walker lived in Alexandria for 35 years, where he was a member of the Rotary Club and a ruling elder in the Westminster Presbyterian Church.

Survivors include his wife, Alice, of Hilton Head Island; a daughter, Meg Roberson of Greenville, S.C., and four grandchildren.