Construction began in March 1913. Her cost was projected to be $118,000. She was 118 feet (36 m) long, with a beam of 25 feet (7.6 m). According to the New York Times her pumps would "normally" project 7,000 gallons per minute. However, "under high pressure", she could throw 13,000 gallons per minute.
The vessel was named after a former Mayor of New York City, William J. Gaynor. Gaynor's daughter Marion launched the vessel, on June 26, 1913, at a ceremony in Elizabethport, New York, attended by other senior officials.
She was put up for sale in February, 1961. She was no longer in operational condition when she was put up for sale.
On January 22, 1916, the freighter Sygna carrying railway supplies from the neutral United States to wartorn Russia, returned to port when crew discovered a serious fire in one of her holds. The William J. Gaynor was assigned to put out the freighter's fire. At the inquiry her officer's praised the freighter's pilot for preventing the vessels from coming to shore, and starting fires there. The Sygna' propeller cut a gash in the William J. Gaynor's hull. She had to call on a "wrecking tug" to open the Sygna's hatch before she could suppress the fire in the hold.
On June 21, 1921, the William J. Gaynor was called to Barren Island in Jamaica Bay when a warehouse belonging to the United States Shipping Board was found to be ablaze. By the time the fireboat arrived land-based firefighters had been unable to prevent the fire from spreading to two of the shipping board's four derelict vessels. The freighters Polar Bear and the City of Omaha were also ablaze. The William J. Gaynor with the assistance of land-based firefighters, and the skeleton crews of two more shipping board vessels, was able to keep the two remaining vessels from burning.
In 1932 Popular Science magazine published a former crew member's account of the William J. Gaynor' fight of a fire aboard a munitions barge, during World War One. The crew member described how, after the fire had been put out, the officer in command of Fort Hamilton, where the barge was being unloaded, praised how he and his colleagues stuck by their stations, and didn't withdraw, when the shells started to explode. He wrote that they took the praise and didn't inform him they had no choice, since they had run aground, and couldn't move until the next tide raised the river's level.
In July, 1958, Otto H. Winderl, the Gaynor's pilot, and Eugene E. Kenny, the Gaynor's Captain, were called to testify as a witness at a Coast Guard board of inquiry into the deadly collision of the freighter Nebraska and the tanker Empress Bay. The tanker burst into flame. The Gaynor and other service vessels had difficulty rescuing survivors. The Nebraska's propeller ripped a large hole in the Gaynor's hull. Two crewmembers died, in the blaze, but 49 others were saved.
In the presence of New York Fire Department officials and a throng of invited guests the fireboat William J. Gaynor was launched successfully at the yard of the New Jersey Dry Dock Company at Elizabethport, N.J., yesterday afternoon. Mayor Gaynor was not present.
Sygna's Captain Finds Blaze in Hold Under Hatches Held Down by Steel Car Bodies. NEUTRALITY GUARD GETS AID Calls Fireboat by Wireless to Meet Burning Ship — Wrecking Tug Opens Way to Flames.
Two of four steamships of the United States Shipping Board moored at Barren Island were set afire shortly after 10 o'clock last night when a blaze which started in a row of two-story buildings, extending three blocks on the east side of the island in Jamaica Bay extended to the two piers to which the vessels were tied up.
High praise was heaped yesterday upon an East River pilot who kept a burning freighter and a blazing tanker in midstream last Wednesday. The praise came from the pilot of the city fireboat William J. Gaynor.