Humboldt (AVP-21) was launched by Boston Navy Yard 17 March 1941; sponsored by Mrs. William T. Tarrant; and commissioned 7 October 1941, Comdr. W. G. Tomkinson in command.

Following rigorous shakedown training off the Atlantic coast, the new seaplane tender sailed from Norfolk 13 May 1942 to join Rear Admiral Ingram's South Atlantic Force on the Brazilian coast. After stops at San Juan and Trinidad she arrived Recife 5 August and began tending the aircraft of VP-83. During the months that followed, these patrol aircraft, operating with ships of the Brazilian and U.S. Navies, patrolled the vital South Atlantic sea lanes and hunted down Axis submarines. Humboldt supplied and repaired seaplanes and, in addition carried aviation gasoline to outlying air bases on the coast while engaging in antisubmarine patrol herself.

While at Natal, Brazil, 28 January 1943, Humboldt was the site of a conference between President Roosevelt returning from the Casablanca Conference, and President Vargas of Brazil. Following this meeting, which helped to achieve even closer cooperation between the naval units of the two countries, the seaplane tender continued to visit isolated ports on the Brazilian coast with supplies establishing a new seaplane base at Aratu in May 1943.

Humboldt headed north 1 July 1943, arriving Boston 17 July to take up new duties in the North Atlantic. Sailing 23 August, the ship carried supplies and parts to the Fleet Air Wings in Newfoundland, Iceland, and Britain. She continued this dangerous duty, often sailing unescorted, into the early months of 1944, occasionally sailing to Casablanca as well. She was at Casablanca in late May 1944 and upon hearing of the torpedoing of escort carrier Block Island and destroyer Barr steamed out to help with survivors and to escort Barr to safety. Humboldt was soon underway again, this time to bring an experienced submarine offlcer to rendezvous with Guadalcanal's hunter-killer group, which had just captured U 505 in an epic encounter 4 June. The seaplane tender continued to bring supplies to squadrons in the Azores and North Africa until 22 March 1945 when she sailed from Norfolk for Brazil.

Returning to her original tending duties in the South Atlantic, Humboldt arrived Recife 5 April and remained on duty until the surrender of Germany, after which she sailed for Norfolk 10 June. She moved to Philadelphia Navy Yard 16 July for conversion to a Press Ship. Reclassified AG-121 30 July 1945, Humboldt was to serve as a broadcast and teletype center for correspondents during the final phases of the war against Japan; but the war ended before her conversion was completed. Arriving Orange, Texas, 22 November 1945, the ship decommissioned 19 March 1947 and entered the Atlantic Reserve Fleet. Loaned to the Coast Guard in January 1949, she now serves at Boston as a weather ship, designated WHEC-372.