William Crowninshield Endicott was born on September 28, 1860 to William Crowninshield and Ellen (Peabody) Endicott, in Salem, Massachusetts. The Endicotts were a prominent New England family, directly related to John Endecott, the Governor of London's Plantation in Massachusetts Bay. William Crowninshield Endicott Sr. had been an Associate Justice of the Supreme Court of Massachusetts, Secretary of War, lawyer, and overseer of Harvard College, among many other roles, while his wife, Ellen Peabody, came from one of Salem's most influential and wealthy families. After attending the Bowditch Grammar School and a year or two of high school, William Crowninshield Endicott Jr. entered Harvard College in 1879. After graduation in 1883, he then began studying at Harvard Law School. He did not finish his legal schooling, but still joined a law office in Salem and passed the bar in 1886. Before beginning a law practice in Salem, Endicott went to Washington D. C. for two years to act a private secretary for his father, then the Secretary of War.

On October 3, 1889 Endicott married Marie Louise Thoron, the daughter of Joseph and Anna (Baker Ward) Thoron. For four years he maintained an office in Boston, until he moved back to Washington D. C. to act as private secretary for Attorney General Richard Olney. In 1897, the couple moved back to Boston, and Endicott discovered that he preferred to manage the care of estates. Over the years Endicott took charge of the care of the family estates, collecting heirlooms, and studying family and Essex County history. He became a trustee and later treasurer of the Groton School, a member of the Essex Institute in 1894, and in 1910 was one of the incorporators and later president of the Society for the Preservation of New England Antiquities. In 1905, he became a trustee, and later an officer and vice-president, of the Peabody Museum in Salem. As a personal friend of Isabella Stewart Gardner, Endicott actively participated in the collecting and placing of art in what became the Isabella Steward Gardner Museum. He was one of the original seven trustees, vice-president, and later president of the Museum until his death. Endicott was also president of the Massachusetts Historical Society for ten years until his death. He was involved in many other institutions as well. Endicott died November 28, 1936.