After the Communists seized power in the wake of the Laotian Civil War, he was the de facto leader of Laos from 1975 until his death.

[3] He attended law school at University of Indochina in Hanoi alongside fellow future revolutionary Nouhak Phoumsavan, but dropped out to fight the French colonialists in Vietnam.

In 1955, he was instrumental in setting up the LPRP at Xam Neua in the north, and subsequently served as the Pathet Lao leader.

At a National Conference of People's Representatives that opened on December 1, Kaysone declared the abolition of the monarchy and the establishment of a republic.

The following day, on a motion by presiding officer Kaysone, the National Conference accepted King Sisavang Vatthana's abdication, abolished the monarchy, and proclaimed the Lao People's Democratic Republic.

According to Vatthana Pholsena, assistant professor of Southeast Asian Studies at the National University of Singapore and author of the book "Post-War Laos", Kaysone was "the top policy maker and a strongman in the LPDR".