Simandjuntak obtained a Bachelor of Economics from Universitas Katolik Parahyangan, Indonesia, in 1973, a Diploma rer-Pol in Monetary Economics and Public Finance from University of Cologne in 1979 and a Doctorate in Philosophy in International Economics from University of Cologne in 1983.

He was a Commissioner of PT Bank Lippo Tbk. He started his career as an Auditor in SGV Utomo from 1973 to 1974, became Head of the Department of Economics Affairs of CSIS from 1984 to 1987, Chairman of the Board of Trustees of CSIS from 1999 to 2005 and Dean of Prasetya Mulya Business School from 1989 to 2003. He serves as a Commisioner at PT Indoritel Makmur Internasional Tbk. He has been an Independent Commissioner of PT Indo Tambangraya Megah Tbk since Jand its President Commissioner since March 2018. Simandjuntak serves as the Chairman of the Board of Management of Centre for Strategic International Studies (CSIS) and Rector of Universitas Prasetiya Mulya. Simandjuntak has been the President Commissioner of PT Indoritel Makmur Internasional Tbk.

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