Phetsarath was born on 19 January 1890 in the Kingdom of Luang Phrabang, Siam, the second son of Oupahat Bounkhong and his second wife, Princess Thongsy.

He established the system of ranks and titles of the civil service, promotion and pension plans, and created a Lao consultative assembly, reorganized the king's Advisory Council.

Phetsarath sent a telegram to all Laotian provincial governors notifying them that the Japanese surrender did not affect Laos' status as independent and warning them to resist any foreign intervention.

On 15 September Phetsarath declared a unified Kingdom of Laos which led the King to dismiss him from his post as Prime Minister on 10 October.

On 10 April 1957, he traveled to Luang Prabang by car and was received by an enormous crowd of citizens, government officials, and members of the police and the army.

On 16 April he paid a courtesy call to King Sisavang Vong and was given back his old title of Uparaja of the Kingdom of Laos.

He was offered an official government residence in Vientiane, but preferred to stay in his villa, Xieng Keo, in Luang Prabang with his Thai consort, Mom Aphiphorn.

In early October 1959 the Phoui Sananikone Government decided to use Phetsarath's official residence in Vientiane as the new prime minister's office.