The ship escorted a convoy from Kwajalein to the naval base at Truk in January 1942, where a sponson was installed for a pair of license-built Type 93 anti-aircraft machineguns on the forward side of the bridge. "Mutsuki" then escorted a troop convoy from Truk to Guam later that month, and then joined the invasion of the Solomon Islands, covering the landings of Japanese forces during Operation R (the invasion of Rabaul, New Ireland and New Britain), and during Operation SR (the invasion of Lae and Salamaua on New Guinea) in January–March. From 28 March to 1 April, she served as the flagship for Rear Admiral Masao Kanazawa, commander of the 8th Special Base Force during the initial occupation of the Shortland Islands and Bougainville in the Solomon Islands. Later that month, the ship supported the occupation of the Admiralty Islands.