Theo Kalionzes

Theo Kalionzes is a Senior Advisor for The Oppenheimer Project and a Nonresident Fellow at the Stimson Center. He brings over a decade of experience in private philanthropy, shaping strategies and making grants to reduce global security risks. 

From 2015 to 2023, Theo was at the John D. and Catherine T. MacArthur Foundation in Chicago. He co-developed the strategy for a MacArthur Big Bet, which delivered $100 million in grants from 2015 to 2020. From 2020 – 2023, Theo helped to build a three-year, $30 million capstone initiative at MacArthur. He served as an Internal Advisor to MacArthur’s Climate Solutions program for many years. MacArthur’s Board approved 200 grants Theo recommended during his tenure, totaling over $80 million in charitable giving.

From 2015 to 2023, Theo led a multidisciplinary convening and grantmaking effort at MacArthur that focused on the climate-nuclear nexus. It supported productive, evidence-based, and forward-looking engagement on the future of nuclear energy as a climate solution. Before MacArthur, he was a program staff member in the International Peace and Security Program at Carnegie Corporation of New York. Previously, Theo was a consultant and speechwriter in the External Relations section of the Comprehensive Nuclear-Test-Ban Treaty Organization, a United Nations family organization based in Vienna, Austria.

He received a Master of Arts in International Policy Studies from the Middlebury Institute of International Studies at Monterey and a Bachelor of Arts in History from the University of California, Berkeley, where he graduated Phi Beta Kappa. He attended Santa Rosa Junior College from 2000 to 2004 when he transferred to UC Berkeley.