Thongduang was born in 1737 in the reign of King Boromakot of Ayutthaya. His father was Thongdi, a Mon noble serving the royal court (posthumously raised to Somdet Phra Prathom Borommahachonnok – "the grand primordial father") who was Phra Akson Sunthonsat (Royal Secretary of northern Siam, Keeper of the Royal Seal). Phra Akson Sunthonsat was also a descendant of Kosa Pan, the leader of King Narai's embassy to the French court.[3][4] His mother, Daoreung (original name Yok), was part-Chinese.[5][6][7] Thongduang had six other siblings.
Thongduang at a young age entered the Royal Palace as one of the royal pages of King Uthumphon, where he met his childhood friend Taksin. In 1757, aged 21, he became a monk temporarily, in accordance with Siamese custom. In 1760, he married Nak, daughter of a town patron in Samut Sakorn. He was later appointed the Luang Yokkrabat (deputy governor) of Ratchaburi Province by King Ekkathat in 1758.