In 1946, he moved to Bojonegoro, where he joined TLRI (Sea Army of the Republic of Indonesia, a non-sailor unit inside the Indonesian Navy, similar to marines) from 1946 until 1948.

In 1952, he enrolled as a student at the Special Infantry Course, and from 1956 until 1957, he studied at the Indonesian Army Command and General Staff College.

After graduating from the college, he became a teacher for military subjects and was involved in the research and development section of the Indonesian Army.

Tambunan and Witono were both commemorated by the Indonesia-Australia Defence Alumni Association with the Sarsono–Tambunan Memorial Lecture Series held in Indonesia and Australia on a quarterly basis.

[5] On 1 November 1968, Tambunan was promoted as a colonel and became the chief of staff of the 1st Military Regional Command/Bukit Barisan, located in North Sumatra.

[6] He was transferred to Java, and in the same year he became the Deputy Governor of Educational Operations in the Republic of Indonesia Armed Forces Academy.

[1][7] On 12 June 1978, Tambunan was officially inaugurated as the Governor of North Sumatra by the Coordinating Minister for Political and Security Affairs Maraden Panggabean.

In 1981, Tambunan enacted a decree that adds 10 subdistricts in North Sumatra, specifically in Binjai, Pematangsiantar, and Sibolga.

[9] In the agricultural sector, Tambunan enacted a decree for the special intensification and extensification (INSUS) for rice production.

[9] Tambunan was also one of the initiators for the establishment of North Sumatra as the third tourist destination in Indonesia after Java and Bali.

[10] To promote ethnic diversity in North Sumatra, Tambunan always began his speech with the greeting "Hoooi!