Robert M. Morgenthau, for more than 40 years the U.S. attorney for the southern district of New York and the longest serving district attorney in Manhattan history, died on Sunday in Manhattan at the age of 99, a few days shy of his 100th birthday. He had been a Vineyard seasonal resident, in Chilmark and West Tisbury, for more than 50 years.

The arrival of Mr. Morgenthau and his 30-foot Brownell bass boat, Souvenir, was awaited every summer at Menemsha, when it came out of winter storage at the Gannon and Benjamin Boatyard in Vineyard Haven. With his Vineyard friends, including the late New York Times journalist Mike Levitas, seasonal resident of Aquinnah, and judge and lawyer Leo Milonas, seasonal visitor to West Tisbury, he would set out from Menemsha for bluefish off Noman’s. After a day on the water, he would gleefully show up at Capt. Everett Poole’s fish market to sell him his catch, until a law went into effect stating that only licensed fishermen could sell to fish markets.

Mr. Morgenthau and his first wife, Martha Patridge, who died in 1972, began summering on the Vineyard in the 1960s, buying property at Quitsa. After her death and his marriage in 1977 to journalist Lucinda Franks, he bought the Toby and Evelyn Kramer house near Middle Road on Music street in West Tisbury. Until recent years, when walking became difficult for him, he was frequently seen with his white poodle ambling on West Tisbury dirt roads. He was always a popular and entertaining dinner guest, spending time with the late writers Lillian Hellman and William Styron, among others.

But he was socially happiest in his simple surroundings of West Tisbury, enjoying the bluefish he and his friends had caught, and steamers from Tisbury Great Pond.

He was born in Manhattan July 31, 1919, a son of Henry Morgenthau Jr., President Franklin D. Roosevelt’s secretary of the treasury from 1934 to 1945, and Elinor (Fatman) Morgenthau. His paternal grandfather, Henry Morgenthau Sr., had been President Woodrow Wilson’s ambassador to the Ottoman Empire (now Turkey) in World War I.

He grew up in Manhattan and on an upstate New York family farm notable for its apples.

He attended the Lincoln School in Manhattan and was a graduate of Deerfield Academy and Amherst College. He was in the Naval Reserve in college and went on active duty in World War II. One of the vessels on which he was serving as a lieutenant was attacked by Nazi torpedo bombers and went down. He was decorated for bravery after swimming for three hours with several shipmates he helped to keep afloat until they were rescued.

After the war, he enrolled in the Yale Law School and graduated in 1948. After practicing law for 12 years, and dabbling in Democratic politics in Riverdale, N.Y., where he lived, in 1960 he became the chairman of Bronx Citizens for Kennedy. Later, as a reward for his efforts, he received his appointment as a United States attorney for the southern district of New York. He then was elected district attorney for Manhattan, retiring in 2009. As district attorney he handled more than three million cases against murderers, muggers, sex offenders, stock manipulators and other miscreants.

He is survived by his wife, Lucinda Franks; five children from his first marriage and two from his second marriage; six grandchildren and three great-grandchildren.

A funeral was held on Tuesday this week at Temple Emmanuel in New York.