Constantino attended the
University of the Philippines where he became the youngest editor of the university's student publication,
The Philippine Collegian. He wrote editorial columns criticizing President
Manuel Quezon, which earned the attention of the President by responding to the article in one of his speeches. It was also in UP where he co-founded the
Alpha Phi Beta fraternity, alongside 17 other students; professor and future Senator
Ambrosio Padilla served as their Charter Adviser. When the
Second World War erupted, Constantino fought in
Bataan and was a member of an intelligence team spying on the Japanese. He also worked as a journalist during the war. At the conclusion of the war, Constantino joined the Philippine Mission to the
United Nations from 1946 to 1949 as its Executive Secretary. He worked as a counselor for the
Department of Foreign Affairs from 1949 to 1951. These exposures to foreign service became the foundations of a book he wrote about the United Nations. ==Academic career==