Francis Van Wyck Mason, 76, a prolific writer of historical novels, mysteries and spy thrillers, drowned Monday while swimming in the Atlantic near his home at Southampton, Bermuda.

Neighbors near the Mason residence saw the body in the sea and brought it ashore. Bermuda police said the Boston-born writer apparently suffered a heart attack, but that an inquest had been scheduled.

Mr. Mason, who had lived on Bermuda for the past 15 years, was at work on his 58th book at the time of his death. It was called "Armored Giants" and was a novel based on the American Civil War naval battle between the Merrimac and the Monitor - the first between iron-clad ships.

Last month, Mr. Mason told a Bermuda newspaper he thought the book was the best he had ever written.

Although his name had not been in the first rank of best-seller authors in recent years, Mr. Mason continued to publish about one book a year. Among his more recent books were "Log Cabin Noble" (1973), "Trumpets Sound No More" (1975), and "Guns for Rebellion" (1977).

In 1938, he published "Three Harbours," the first volume of a trilogy about the American Revolution and his first historial novel. It was an instant success and sold more than 100,000 copies in hard-cover editions.

Before turning to historical novels, he wrote several spy stories with international settings and with "Capt. Hugh North" as their hero, Mr. Mason described his work at that time as consisting of "Foreign Legion stories, juveniles, historical romances, war stories, flying stories, and many types of adventure-action stories."

Mr. Mason's own life was not without its elements of adventure.

Having been born in Boston, he spent his first eight years in Berlin and Paris, where his grandfather was the U. S. consul. He spoke fluent Spanish and French and some German and served as an interpreter with the American Expeditionary Force in France during World War I.

He graduated from Harvard College in 1924. Until 1927, he headed an importing company he had founded. In this period, he played much polo, he later recalled. He also was a member of Squadron A, the fashionable horse-cavalry unit of the New York National Guard. he traveled widely over the years in Europe, Central America, the Caribbean, and North Africa.

He published his novel, "Seeds of Murder," in 1930, but by his own account it was a year or so later that he "entered the literary business seriously."

During World War II, he was a colonel attached to Gen. Dwight Eisenhower's headquarters in Europe.

In 1927, Mr. Mason married Dorothy Lousie Macready, who died in 1957. They had two sons, F. Van Wyck II, and Robert Ashton. In 1958, he married Jeanne-Louise Hand, who had been his secretary and who survives him.