SC 21 was built at the New York Shipyard in Brooklyn. This chaser was placed into commission on 20 September 1917, under the command of Lt. Alfred Y. Lanphier, who would later take command of the USS Patterson Group. SC 21 was assigned to the Second Naval District, New London, CT, and worked with the experimental group there.
On 6 May 1918, Lt. (j.g.) Henry Hartley, USN, was relieved of command of SC 253 and took command of SC 21.
From a collection of photos of a crewman of SC 254. Thanks to Laura Hawkins for submitting the images to The Subchaser Archives.