His activities as a politician started in Jong Sumatranen Bond. Afterwards, he joined "Perhimpunan Pemuda Indonesia". While in the "Rechts Hoge School" his professors disliked Assat's political activities and Assaat failed his exams. He then joined the Indonesia Party (Partindo) with Adnan Kapau Gani, Adam Malik, and Amir Sjarifoeddin. After Indonesian independence, he was elected as head of the Central Indonesian National Committee (KNIP) until Sukarno was captured by the Dutch. Assaat was named President of the Provisional Government of Indonesia until Sukarno's return. In December 1948, the Dutch launched their second "Police Action" and focused their attack on Yogyakarta. Assaat was captured by the Dutch army and sent to exile on Bangka Island along with other nationalist leaders.