Simatupang was born in Dairi, North Sumatra, then part of the Dutch East Indies to a Batak Protestant family.

[4] He notes that other Batak youth called them De Drie Musketiers, that they were often together attending lectures by people such as Sutan Sjahrir and collecting books about independence movements in other countries.

[5] Like Army Chief of Staff General Nasution, he was an "administrator", committed to the reduction in size of the armed forces after independence had been won to bring about a professional military.

He opposed efforts by a disgruntled rival, Colonel Bambang Supemo, to replace Nasution, but was himself criticized for apparent political bias after articles he wrote in 1952 were perceived as favoring the Socialist Party of Indonesia (PSI).

After the incident on 17 October 1952, in which the army brought demonstrators and troops to the Merdeka Palace to persuade President Sukarno to dissolve parliament, Simatupang's days were numbered, and on 4 November 1953, his post as chief of staff was abolished, effectively dismissing him.