Theo Kalionzes is a Nonresident Fellow at the Stimson Center with the Partnerships in Proliferation Prevention program. 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 the Nuclear Challenges Big Bet, which delivered $100 million in grants to the nuclear policy field from 2015 – 2020. From 2020 – 2023, Theo helped to build a three-year, $30 million capstone initiative at MacArthur. For many years he served as an Internal Advisor to MacArthur’s Climate Solutions program. 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, 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 the Santa Rosa Junior College from 2000 to 2004 when he transferred to UC Berkeley. Theo serves on the advisory board of the Outrider Foundation.