He studied medicine at the STOVIA medical school in Batavia, where from 1917 to 1919 he was the founding chairman of the Jong Sumatranen Bond, a Sumatran nationalist organization.

He later worked as a doctor in Sulawesi and Batavia, then returned to Medan where he became a highly respected surgeon and wrote medical books in the Malay language.

[5] On 15 August 1945, the Japanese surrendered, and two days later, Indonesian nationalist leader Sukarno proclaimed the independence of Indonesia in Jakarta.

There were two outcomes from this meeting: a statement asking the people to stay calm, and the establishment of a committee led by Mansur and the Sultan of Langkat tasked with explaining to the victorious Allies why they had needed to cooperate with this Japanese.

Rumors soon spread that this was in fact a committee established to welcome the returning Dutch colonial forces, or even to seize power in the meantime.

The sultans and the Sumatran elite, anxious to protect their own status and interests, had been hoping for the return of the Dutch and for what they saw as the normal state of affairs to be restored.

One speaker read out a petition addressed to the Dutch Lieutenant Governor General asking him to recognize such a state within a federal Indonesia.

This caused discontent among the former aristocrats, and at the same time Mansur was unable to attract support from immigrant communities, particularly the ethnic Javanese, who still mistrusted him.