Crowninshield was born on October 13, 1867 in New York City. He grew up in Marblehead, Massachusetts, into the wealthy Crowninshield family with long-standing ties to the sea. The family estate, Crowninshield House, was built by his father in 1870. His father was Benjamin Williams Crowninshield (1837–1892) and mother was Katherine May Bradlee (1844–1902). His younger brother Francis Boardman Crowninshield (1869–1950), married heiress Louise Evelina du Pont (1877–1958).
Through his paternal grandmother Sarah Gool Putnam (1810–1880), he was distant cousin of architect J. Pickering Putnam (1847–1917). His great-grandfather Benjamin Williams Crowninshield (1772–1851) had served as Secretary of the Navy, and his great-granduncle George Crowninshield Jr. (1766–1817) built the first luxury yacht in the United States, Cleopatra's Barge in 1816. His cousin once removed was Frederick Josiah Bradlee, Jr. (1892–1970) whose son was Benjamin Crowninshield Bradlee (1921-2014). Because of his many relations, he was known as "B.B." Crowninshield.
He attended Prince School in Boston and graduated from St. Paul's School in Concord, New Hampshire. He attended Massachusetts Institute of Technology in 1885, but transferred to Harvard University, where he graduated in 1890.
After graduating from Harvard, he speculated in real estate before signing on as a draftsman with John R. Purdon, a respected yacht designer in Boston with several knockabout designs to his credit. Crowninshield struck out on his own 18 months later, starting a yacht design and brokerage firm which quickly prospered. He would rise to become one of America's most respected yacht designers during a period which is now regarded as the golden age of American wooden yacht design.
He designed the schooner Adventuress, which was launched in 1913 and has been named a National Historic Landmark. Crowninshield is now best remembered for his working schooners and his America's Cup contender Independence (George Lawley & Son shipyard, 1901) for Thomas W. Lawson. He was also responsible for designing the Dark Harbor sloop.
He designed the unique seven masted schooner Thomas W. Lawson, named for his patron.
He married Priscilla Janet McPhail (1869–1915) on May 12, 1900. In 1902, he was fined and was sued for $10,000 for assaulting Adolphus G. McVey, the yachting editor of the Boston Herald, for a remark about his wife. After nine years, he was ordered to pay $448. His wife was found dead in a bathtub of a Boston hotel on October 8, 1915. She was checked in as "Mrs. Bowdoin", but the death was ruled accidental. Together, they were the parents of:William Widlar Crowninshield (1909–1987), who married Muriel Adese Longton (1915–2002).
In 1916, he married Laura A. Wildar (1877–1952), daughter of Leonard John Wildar. Laura had previously been married to Charles T. Long, whom she divorced in August 1915.
Crowninshield died August 12, 1948 in Marblehead.Schooner Adventuress