Following shakedown in the Atlantic, "Hollis" made two escort voyages along the East Coast and then reported to Quonset Point, R.I., to assist in sonic research. The aim was to find countermeasures for the German acoustic torpedo, and the destroyer escort remained on this important duty until 28 May, when she sailed to Casablanca in an aircraft carrier screen. Returning to New York on 17 June, "Hollis" was soon at sea again, this time as part of an escort and hunter-killer unit. She operated from July to mid-August escorting convoys in the Mediterranean, and escorted a convoy to the southern France invasion area on 15 August as Allied troops stormed ashore. In the months that followed, as the offensive gained momentum, "Hollis" continued to act as an escort in the Mediterranean, ensuring the flow of vital supplies and men. She sailed for the United States on 28 December, and arrived on 18 January to undergo conversion to a high-speed transport at Philadelphia Navy Yard.