He was born in Asan, South Chungcheong province, Yun graduated from the University of Edinburgh in 1930. He entered politics after Japanese occupation ended in 1945, with Syngman Rhee being his mentor. In 1948, Yun was made mayor of Seoul by Rhee. A year later, he was made the Minister of Commerce and Industry. Soon, he started to disagree with Rhee's authoritarian policies. He then served as president of the Red Cross Society, before being elected to the National Assembly in 1954. A year later, he founded the opposition Democratic Party along with several others.