She played an important role in the Battle of Shanghai in the opening stages of the Second Sino-Japanese War, and later covered the landings of Japanese forces in southern China.

At the time of the attack on Pearl Harbor, Sendai was engaged in escorting transports carrying Lieutenant General Tomoyuki Yamashita and the Japanese 25th Army to invade Malaya.

At 23:45 on 7 December 1941, Sendai and her destroyer squadron (Ayanami, Isonami, Shikinami, and Uranami) commenced a bombardment of Kota Bharu, Malaya.

The report was received by Sendai, which relayed the message to Vice Admiral Jisaburō Ozawa aboard his flagship, the heavy cruiser Chōkai.

On 19 December 1941, off Kota Bahru in the South China Sea, Royal Netherlands Navy submarine HNLMS O 20 sighted Sendai escorting the second Malaya Convoy's 39 transports.

On the fifth run, on 26 January, Sendai and her convoy were attacked by the destroyers HMS Thanet and HMAS Vampire about 80 nmi (148 km) north of Singapore in the Battle off Endau.

From February through March, Sendai was assigned to cover Japanese landings in Sumatra, and in sweeping the sea lanes and the Strait of Malacca for British and Dutch vessels escaping from Singapore.

At the end of March, Sendai covered the landing of one battalion of the IJA's 18th Infantry Division at Port Blair, Andaman Islands.

However, with American landings on Guadalcanal, the planned Indian Ocean operations were cancelled and Sendai was sent to Makassar, Davao and Truk instead, to escort troop convoys to Rabaul, New Britain and Shortland, Bougainville.

The Japanese force included the cruisers Myōkō, Haguro, Sendai and Agano with destroyers Shigure, Samidare, Shiratsuyu, Naganami, Wakatsuki, Hatsukaze.