He was awarded the Navy Cross for his role in protecting convoys against German submarines as commander of the USS Lamson.

In 1945, he travelled to Tinian as the representative of the Military Policy Committee, and coordinated preparations for the atomic bombing of Hiroshima and Nagasaki with senior army and navy commanders in the Pacific.

His citation read:The President of the United States of America takes pleasure in presenting the Navy Cross to Lieutenant Commander William Reynolds Purnell, United States Navy, for distinguished service in the line of his profession as commanding officer of the USS Lamson, engaged in the important, exacting and hazardous duty of patrolling the waters infested with enemy submarines and mines, in escorting and protecting vitally important convoys of troops and supplies through these waters, and in offensive and defensive action, vigorously and unremittingly prosecuted against all forms of enemy naval activity during World War I.

In Australia, Purnell became chief of staff to Vice Admiral William A. Glassford, the commander of US Naval Forces, Southwest Pacific.

He too left Australia in June 1942, for duty in Washington, D.C., in the Office of the Commander in Chief, United States Fleet, Admiral Ernest J.

His citation read:The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Navy Distinguished Service Medal to Rear Admiral William Reynolds Purnell, United States Navy, for especially meritorious service as Chief of Staff to Commander in Chief, Asiatic Fleet, and Commander U.S.

[11] In February 1945, Purnell arranged for Commander Frederick Ashworth to go to Guam to brief Fleet Admiral Chester Nimitz on the project.

[12] Tinian's harbor suffered from congestion due to extensive base development work, and ships sometimes took months to unload.

The director of the Manhattan Project, Major General Leslie R. Groves, Jr., went to Purnell, who arranged for an order from King to Nimitz stating that all shipments related to the Manhattan Project and the 509th Composite Group had to be unloaded immediately on arrival, regardless of the consequent disruption to the port's operations.

[13][14] Problems like this underlined the value of having high-ranking officers on hand to deal with local commanders to make decisions on the spot if necessary.

[15] Along with Captain William S. Parsons, the director of Project Alberta, they formed what became informally known as the "Tinian Joint Chiefs".

Parsons agreed to work the assembly team around the clock to get the Fat Man bomb ready by the evening of 8 August.