[5] In 1950, Siagian joined LEKRA and became an influential member of the Indonesian Communist Party's cultural organisation.

Between 1955 and 1965, he wrote and directed 13 feature films, making him one of the most prolific indigenous Indonesian filmmakers prior to the General Suharto's anti-communist crackdown of 1965.

His films, that he often wrote himself, focused on the weak and powerless, providing rare critical insights into society.

"[3] Daerah Hilang/The Lost Area (1956) that follows 24 hours in life of a released prisoner was heavily censored by the Indonesian government for its socially critical scenes.

Despite the renown and acclaim of many of his films the majority of them, and others of other Leftist filmmakers have been lost or destroyed during the New Order regime.