Japanese forces invaded Thailand early on the morning of 8 December 1941, shortly after the attack on the United States at Pearl Harbor, Hawaii.

The Prime Minister, Field Marshal Plaek Phibunsongkhram, ordered a ceasefire at noon, entering into an armistice that allowed the Japanese to use Thai military installations in their invasion of Malaya and Burma.

Northeastern populist politicians like Tiang Sirikhanth and Bangkok newcomers like Sanguan Tularak were not people the aristocratic Seni preferred to associate with.

The regent's looming presence and overarching authority rankled the proud, thin-skinned Seni, fuelling a personal animosity that would poison Thailand’s postwar politics.

The Pramoj brothers subsequently joined the newly formed Democrat Party in 1946, which was for the most part made up of royalists and conservative reactionaries.

On Tuesday, 14 June 1949, in a lecture delivered before the Siam Society, Seni pleaded, "[I] happen to belong to that peculiar species known as politicians who are in the incorrigible habit of attempting to accomplish the impossible."

Word had gotten around that he and his brother had been "getting up a little English translation of some of King Mongkut's public papers and private correspondence...without actually putting it to a final execution."

With regard to the 1944 semi-fictionalized biographical novel Anna and the King of Siam and the 1946 Hollywood film of the same title, Seni quoted several acts and judicial decisions that gave the lie to the fiction.

He served again briefly as prime minister from 15 February to 13 March 1975, when he was defeated and replaced by his younger brother, Rajawongse Kukrit Pramoj.

After a right-wing backlash against leftist student demonstrators culminated in the Thammasat University massacre on 6 October 1976, the military forced him out of office and installed hard-line royalist Tanin Kraivixien as premier.