In 1943, along with his father, Leone was among the founders of the Christian Democracy (DC), the PPI's heir led by Alcide De Gasperi.
After two years, in 1945, he was elected DC's provincial secretary for Naples, immediately becoming one of the leading figures of the party.

In 1946, Leone was among the main supporters of the "neutrality" in the 1946 institutional referendum, in which Italians voted to abolish the monarchy of the House of Savoy.
In the same year, he was elected with nearly 32,000 votes to the Constituent Assembly for the constituency of Naples–Caserta.
As a prominent jurist, he was appointed in the commission with the aim of drawing up the new republican constitution.

In April 1948, he was elected to the Chamber of Deputies with 60,000 votes.
Even as a deputy, he continued working as a lawyer and teaching at the university, considering for a long time these occupations as priority aspects of his life.
Also for this reason, according to some testimonies, he often expressed the desire not to take on government positions.
Being able to count on his own reliable and territorially rooted electorate, Leone practically never conducted a real party activity.
He remained, therefore, substantially foreign to the large and small factions in which the DC quickly split up, although he could ideally be considered close to the party's conservative wing.
In 1943, along with his father, Leone was among the founders of the Christian Democracy (DC), the PPI's heir led by Alcide De Gasperi. After two years, in 1945, he was elected DC's provincial secretary for Naples, immediately becoming one of the leading figures of the party.
In 1946, Leone was among the main supporters of the "neutrality" in the 1946 institutional referendum, in which Italians voted to abolish the monarchy of the House of Savoy. In the same year, he was elected with nearly 32,000 votes to the Constituent Assembly for the constituency of Naples–Caserta. As a prominent jurist, he was appointed in the commission with the aim of drawing up the new republican constitution.
In April 1948, he was elected to the Chamber of Deputies with 60,000 votes. Even as a deputy, he continued working as a lawyer and teaching at the university, considering for a long time these occupations as priority aspects of his life. Also for this reason, according to some testimonies, he often expressed the desire not to take on government positions. Being able to count on his own reliable and territorially rooted electorate, Leone practically never conducted a real party activity. He remained, therefore, substantially foreign to the large and small factions in which the DC quickly split up, although he could ideally be considered close to the party's conservative wing.
Giovanni Leone