During this time, he spent short terms studying history, constitutional law and English literature at Oxford and Cambridge.

Upon his return to Peru, he married Sofía Yrigoyen and was one of the founders of the National Democratic Party led by José de la Riva Agüero.

In 1919, President Pardo appointed him Minister Plenipotentiary to Uruguay but shortly afterwards he resigned because of the coup d'état led by Augusto B. Leguía.

After his time abroad, Belaúnde returned to Peru and was reincorporated to the University of San Marcos as professor of constitutional law.

That same year, Belaúnde spent a term at Johns Hopkins University, where he delivered the prestigious Albert Shaw Lectures on Diplomatic History.

Belaúnde returned to Peru in 1930 when a military coup led by Commander Luis Miguel Sánchez Cerro overthrew the Leguía's regime.

A new interim junta presided by David Samanez Ocampo created a commission to draft a reform project to the Leguiist Constitution and Belaúnde was included as one of the ten members.

Belaúnde was even denied his tenureship as professor of modern history, which caused indignation in certain academic circles and inevitably led to his definitive departure from San Marcos.

Among the positions Belaúnde occupied throughout his professional career were: Secretary of the File of Limits of the Ministry of External Relations of Peru, becoming Secretary in the Diplomatic Mission to Spain and Argentina, Consultant in the bordering negotiations with Brazil, Chargé d'affaires in Germany (1914) and Bolivia (1945), Plenipotentiary Secretary in Uruguay in 1919, Colombia in 1934 and in Switzerland in 1936, President of the Peruvian delegation before the League of Nations then subscribing the San Francisco Charter, which gave origin to the Organization of the United Nations, He was also President of the General Assembly of the United Nations (1959), Rector pro tempore (1946–1947) and finally Rector of the Catholic University of Peru (1965).