Barnett was launched in 1928 as the passenger ship Santa Maria by the Furness Shipbuilding Company of Haverton-on-Tees, England for the Grace Line.

[2] Among those survivors were Lexingtons executive officer, Commander Morton T. Seligman, Chicago Tribune war correspondent Stanley Johnston, and dive bomber pilot Robert Dixon, who sent the most remembered message of the Coral Sea battle: "Scratch one flattop.

"[2] During the trip, Dixon observed Seligman sharing classified message traffic with Johnston including decrypted information about Japanese planning for the Battle of Midway.

[2] Johnston's Chicago Tribune article following the battle included information potentially revealing Japanese naval codes were no longer secure.

[2] Japanese cryptography remained unchanged following the article, so Seligman's punishment was a permanent ban on promotion to avoid possible publicity about more formal proceedings.

Assignment of the 1st Marines to the Guadalcanal landing occurred after Barnett had been commercially loaded to cram the most material into available cargo holds.

Barnett was anchored off Lunga Point offloading when accompanying USS Alchiba (AKA-6) was torpedoed by midget submarine Ha-10 on 28 November 1942.

Barnett loaded troops of the 45th Infantry Division for a Chesapeake Bay landing exercise at Cedar Point, Maryland, on 12 April.

Barnett sailed from Norfolk, Virginia, on 10 May 1943 with convoy UGF 8A as flagship of the Third Transport Division commanded by Commodore Campbell Edgar, escorted by USS Savannah (CL-42) and 12 destroyers, and arrived in Oran, Algeria, on 23 May 1943.

On 5 July 1943 Barnett sailed with convoy NCF 1 carrying the 26th Regimental Combat Team with General Theodore Roosevelt Jr.[9] to the Sicilian occupation.

Barnett left Oran on 30 September with 1,011 signal, police, and ordnance specialists in convoy NSF 4 and arrived in Naples, Italy, on 6 October.

Barnett then made several trips shuttling troops between Gourock and Plymouth, England, and conducting landing exercises in Start Bay in preparation for the invasion of Normandy.

[8] After a month of shipyard overhaul at Norfolk, Barnett loaded troops at Newport, Rhode Island, in December, and transported them via the Panama Canal to Long Beach, California, in January.