As Sino-Japanese relations broke down, Mr Tan Kah Kee was increasingly involved in mobilizing the Chinese community against Japanese aggression. He was very active in the Shantung Relief Fund and was later appointed as the chairperson of the China Relief Fund Federation in Southeast Asia and he raised millions of dollars from the overseas Chinese community in Southeast Asia for the benefit of war-torn refugees in China. Under his leadership, the Shantung Relief Fund turned into a Japanese goods boycott movement with its headquarters in Ee Ho Hean Club. Tan also formed the National Salvation Movement, an anti-Japanese group in Malaya and Singapore.

After 1936, Tan led the Singapore China Relief Fund and the South Seas China Relief Fund Union, mobilizing Southeast Asian Chinese to contribute financially to the war against Japan. The Singapore China Relief Fund was particularly impressive as it consisted of over 300 branches and mobilized over 300,000 Chinese in Singapore to ensure the widest and deepest reaches into the Chinese community. The fund also contributed to the 400 million Chinese yuan donated by overseas Chinese in Southeast Asia between 1937 and 1942.

Tan's ability to mobilize the masses did not escape the British authorities in Singapore. British Governor Sthenton Thomas requested for Tan's leadership in the Singapore Chinese Mobilization Council to assist in the militia and civil defence of the island. In addition to assisting in manpower mobilization, the British authorities also asked Tan for help in selling war bonds to raise money for the war efforts. He was approached to sell $40 million worth of bonds that had been allocated to Malaya. Tan himself bought $100,000 worth of bonds out of patriotism and through his efforts and those of other personalities within Malaya, the British eventually sold $15 million worth of bonds, a remarkable sum in view of the harsh times then.