The second ship to be so named by the Navy, Buckthorn (YN-9) was laid down on 5 December 1940 at Alameda, California, by the General Engineering & Drydock Co.; launched on 27 March 1941; sponsored by Mrs. Rosalie Benton Day; and placed in service at the Mare Island Navy Yard on 16 September 1941, Lt. Lewis Williams, USNR, in charge. That same day, the ship departed the Mare Island Navy Yard. She arrived at San Pedro, California, on the 20th and reported for duty as a unit of the net defenses and inshore patrol. She spent the next 21 months tending nets and conducting patrols at San Pedro. During that time, on 9 December 1942, Buckthorn's status was changed from in service to in commission.