Leo Mates graduated from a high school and was a student of technical sciences at University of Zagreb. During high school Mates was involved in the revolutionary youth movement and, later, during his studies, he was one of the leaders of the student movement at the University of Zagreb. Mates became a member of the Central Committee of the Union of Communist Youth of Yugoslavia in 1937, and secretary of the Provincial Committee of the Union of Communist Youth for Croatia. Following the occupation of Yugoslavia, in April 1941, as a member of the Communist Party of Yugoslavia Local Committee for Zagreb, Mates managed the printing of illegal journals until early 1942. Then, Mates joined the National Liberation Army of Yugoslavia, where he held several important posts. After the Second World War, Mates was Editor-in-Chief of Tanjug and later he was a successful diplomat – advisor to the Embassy of the FNR Yugoslavia in London, Ambassador of Yugoslavia in the United States of America and Permanent Representative of Yugoslavia to the United Nations. He was Secretary General of the President of Yugoslavia (1958–1961), and subsequently Assistant Secretary of Foreign Affairs. Leo Mates was recognised as a well-known expert for international relations; he authored several books, among which are the most notable Non-Alignment and Superpower and Weapon Policy. Mates was distinguished by several high Yugoslav and foreign honours.