Laurens M. Hamilton, 77, a former resident of Fauquier County, Va., and a grandson of financier J. Pierpont Morgan, died Wednesday of a heart ailment at his home in St. Petersburg, Fla.

Mr. Hamilton was also the great-great-grandson of Alexander Hamilton, first secretary of the Treasury under George Washington. He was born in Tuxedo, N.Y., and attended the Groton School in Massachusetts. He entered the Army in 1918, serving as a second lieutenant in the cavalry for two years before returning to New York.

He worked for the Department of Commerce in New York and later became a member of the New York Store Assembly from Rockland County.

Mr. Hamilton moved to Fauquier County in the mid-1940s and was elected justice of the piece in 1947. Later he was coordinator of civil defense in the county. He moved to St. Petersburg five years ago.

In 1957, Mr. Hamilton represented the Hamilton family on the Alexander Hamilton Bicentennial Commission. He founded the Society of Friends for the Bind on the island of Nevis, West Indies, the birthplace of Alexander Hamilton.

Mr. Hamilton was a 32d degree Mason. He also was a member of the Army & Navy Club of Washington and a former chairman of the board of trustees of the Aramaic Bible Foundation and of the COmmittee on Electoral Reform of the American Good Government Society.

Survivors include his wife, Mary Anne, of the home; three stepsons, Leo Clark, of St. Petersburg, John Clark, of the home, and Michael Clark, of Washington; two stepdaughters, Kathryn Clark, of Washington and Anne Clark, of the home; a brother, Pierpont Morgan Hamilton, of Santa Barbara, Calif., and a sister, Mrs. W. Randolph Burgess, of Washington.

The family suggests that expressions of sympathy be in the form of contributions to the Society of Friends for the Blind, Nevis, West Indies.