Phat was born on 6 February 1878, near Wat Chakkrawat in what was then Phra Nakhon province [th] (now Bangkok) as the eldest son of a total of 8 siblings, his father being Colonel Luang Rit Naiwen (Phut Thephatsadin na Ayutthaya), who was a half-brother of Chaophraya Thammasakmontri.

After the death of his father, who was then deputy commander of the Army War Department, Chulalongkorn sent Phat to for studies in France and he later transferred to a Royal Military Academy in Brussels, Belgium, returning in 1902.

After returning from the war, he resigned from his military position and was transferred to the civil service, with the noble title Phraya Thephatsadin, serving as the samuhathesaphiban or royal commissioner in the monthon of Nakhon Sawan and Ratchaburi.

Throughout his government service, Thephatsadin was reputed to be very loyal to the monarchy and even had a personal friendship with the King, with a tale recounting that Vajiravudh once shared a coconut he was drinking from with him.

Over 51 suspects were arrested in the early morning of 29 January 1939, and a special court was established to try them, which sentenced 21 prisoners to death, including Thephatsadin and two of his sons.