The most important change in Thailand's financial system was engineered under the visionary and wise leadership of Prince Mahisara Rajaharudaya (H.R.H. Prince Mahisorn), a brother of King Chulalongkorn Rama V. He realized that it would be difficult for Thailand to develop its economy without the firm foundation of a national banking system. His approach was to cautiously set up a small commercial bank as a pilot project, naming it the "Book Club".

The Book Club began its life on 4 October 1904 in a shophouse on Ban Moh Road. After the heartening performance of the Book Club, on 30 January 1907, H.M. King Rama V granted royal permission to set up the bank with an initial capital of 3,000,000 baht. The decree was announced in Royal Gazette, Siam Commercial Bank Limited thus officially began operations as a full-fledged commercial bank. In the realization of H.R.H. Prince Mahisara’s vision, the Bank would soon grow to become a "prototype for Thai bank", and help to achieve Thailand’s financial independence, immensely benefiting the Kingdom and its people.