Tan was one of the prominent overseas Chinese to provide financial support to China during the Second Sino-Japanese War. He organised many relief funds under his name, one of which alone managed to raise ten million Straits dollars in 1937. He was also a participant in the Legislative Yuan of the Nationalist government in Chongqing. After the Japanese invaded and occupied Malaya and Singapore in 1942, they deemed these contributors "undesirable" and conducted a systematic extermination of anti-Japanese elements in Singapore through the Sook Ching Massacre. Tan survived because he escaped from Singapore before it fell to the Japanese, and went into hiding in Malang, a town in East Java province, Indonesia. He strongly rejected proposals to attempt to negotiate with the Japanese and regarded such attempts as characteristic of a "hanjian" (a Chinese term for race traitor). He also attempted to dissuade Wang Jingwei from such activities. He exercised considerable effort against the governor of Fujian Province, Chen Yi, for perceived maladministration.