He chaired Prisdirektoratet over two periods, a total of twenty years. A special law from 1945, "Lex Thagaard", was named after him. He represented Norway internationally in Organization of European Economic Cooperation, Organization of European Cooperation and Development and General Agreement of Tariff and Trades.
During the occupation of Norway by Nazi Germany Thagaard was a member of "Kretsen", a leading body of the Norwegian Resistance Movement.