After the first world war he joined the Partito Popolare Italiano of Don Luigi Sturzo. This party was banned by Mussolini in 1926. In 1944 he became a member of theDemocrazia Cristiana, the Italian Christian Democratic party.
In June 1955 he formed the seventeenth post-war cabinet with Christian Democrats, Liberals and Socialists. This Government fell in may 1960, when the Social DemocratGiuseppe Saragat, the Foreign Minister, stepped out of the Government.
In the Cabinet of Amintore Fanfani (1958-1959) was Segni Deputy Prime Minister and Defence Minister. On February 15, 1959 he was again premier, now of a single cabinet which only joined the Democrazia Cristiana. A year later, on 24 February 1960 came across this Government. from 1960 to 1963 was Segni Minister of Foreign Affairs.