During the summer of 1940, after shakedown and an extended training period, Mayrant escorted her Commander in Chief, Franklin D. Roosevelt, on a tour of east coast defenses. Later on in the year, again escorting the President, she visited island bases newly acquired from Great Britain under the “destroyers for bases” agreement.
The following spring, 1941, as U.S. involvement in European hostilities increased, her Navy expanded its efforts to keep the sea lanes open. In May, the limits of the neutrality patrol were extended and the Navy gradually expanded its responsibilities for transatlantic convoys. By September, it was officially responsible for protecting them as far as Iceland, lengthening the patrols of the Support Force, Atlantic Fleet, which had been assigned the task.
Mayrant, on duty with that force, operated off Newfoundland during the spring and summer. In August, she stood-by during the Atlantic Charter Conferences and, at their conclusion, escorted HMS Prince of Wales, carrying Prime Minister Churchill, to Great Britain. (continued)