Following shakedown off Bermuda, "Monssen" steamed north to Boston, Massachusetts, to join the new cruisers , , and and escort them to San Diego. From San Diego, she screened Carrier Division 26 (CarDiv 26) to Pearl Harbor, arriving 8 May for rehearsals for the Marianas campaign. On 30 May, the destroyer sailed with Task Group 52.16 (TG 52.16) for Eniwetok, whence she continued on to Saipan, arriving on 15 June. By dawn, she had commenced preparatory firing against Japanese positions on Green Beach 1. Close fire support was started with the initial landings on Saipan and kept up, almost continuously, until noon, 17 June. Included in her accomplishments during that period was her assistance in the breaking up of a large-scale enemy tank and troop counterattack, south of Garapan, at dawn on the 16th.