Leonis was laid down 21 November 1942, as liberty ship SS Key Pittman, MCE hull 512, by Permanente Metals Corporation, Yard No.

[4] The arrival of Key Pittman back in Milne Bay on 17 June signified the end of Operation Lilliput which had before that time largely involved Dutch ships.

[6] Following training and repairs, the cargo ship departed Pearl Harbor 20 August, to join the forces preparing for the Palau Islands campaign.

[6] After loading Marines and other passengers at Peleliu, the Russell Islands, Tulagi, and Guadalcanal, the cargo ship sailed for the United States, arriving San Francisco 18 December.

Reloading with battle supplies, Leonis departed San Francisco 11 January 1945 to join the Western Pacific Forces as a fleet issue ship.

[2] The ship was subsequently laid up in the National Defense Reserve Fleet, James River Group, Lee Hall, Virginia.