Fearing an outbreak of war in Europe, in July 1939 Tun Tan Cheng Lock ordered him and his two sisters to leave London and return to Malacca.

On 1 September 1939, Adolf Hitler invaded Poland, an event that marked the start of World War II.

[1] In March 1968, Tan proposed setting up the Tunku Abdul Rahman College for Chinese youths who would otherwise be denied an opportunity to tertiary education.

He was also the chairman of United Malacca Rubber Estates, and sat on the boards of a number of companies, including Unitac, Siemens, Pacific Bank, Highlands & Lowlands, and Guardian Royal Exchange Assurance.

[1] Tan Siew Sin died on 17 March 1988 in Kuala Lumpur, and was buried in the family burial ground in Malacca.

[9][10] His widow, Catherine Lim Cheng Neo, whom he married on 8 February 1947 was an active campaigner for family planning.

At Tunku Abdul Rahman University College 's Main Campus in Kuala Lumpur there is a new building named after him, known as "Bangunan Tun Tan Siew Sin".