Post shakedown training completed in time to participate in the Marianas campaign, "McGowan" arrived at Roi, Kwajalein Atoll, 31 May 1944. Ten days later she sailed with Task Group 52.17 (TG 52.17) for Saipan. On 14 June she screened the bombardment ships. The next day, during the invasion of Saipan she added fire support to her duties, disposing of a fuel dump and artillery emplacements endangering forces on the beach. As the beachhead expanded, "McGowan" continued to support the assault forces with counter-battery and harassing fire until the 23d, when she retired to Eniwetok. Next assigned to TG 53.1 she screened the transports carrying troops to Guam, remained through the initial landing operations, and then set course back to Saipan. There she rejoined TG 52.17 for screening and fire support missions during the Tinian phase of the conquest of the Marianas.