n March 1943, Andreotti met Alcide De Gasperi at Giuseppe Spataro's home, who asked him to work alongside Guido Gonella on the editorial staff of "Il Popolo," the future newspaper of the Christian Democrat party. He developed an increasingly close relationship with De Gasperi, acting as a liaison between "Il Popolo" and the Trentino politician, becoming his unofficial voice. This meant following both international and domestic politics, since De Gasperi had been both Foreign Minister and Secretary of the DC since December 1944.

When De Gasperi assumed the leadership of the government, Andreotti's role became more important. Following the Republic's victory in the 1946 referendum, Andreotti accompanied De Gasperi to King Umberto II. The Trentino native informed the King that he would assume power as the provisional head of state, so Umberto II left for exile in Portugal before the recount to avoid further bloodshed in Italy.

Andreotti supported the De Gasperi government's actions on both the Peace Treaty and the Trieste question, accusing Yugoslavia of refusing to engage. He also defended the government from attacks by the PCI, which was attempting to exploit discontent to regain votes after its June defeat. The dynamics of the Cold War brought the DC into ever greater conflict with the PCI-PSI coalition, and within the Christian Democrat Party, including Andreotti, there was a push to exclude Marxists from the government. De Gasperi was persuaded in May 1947 and formed a government without the PCI and PSI, with Andreotti serving as Undersecretary to the Presidency.

In this capacity, Andreotti's responsibilities increased. Following the fateful 1948 elections, he served as De Gasperi's agent in the election of the President of the Republic and in the formation of the new government. He consulted Einaudi, the DC candidate for President of the Republic, who was, in fact, elected on the fourth ballot. Andreotti had accumulated considerable responsibility and power, a fact noted by many within the DC; some even believed he controlled De Gasperi and decided on positions and appointments. For Spataro, Andreotti was entrusted with too many formal and informal duties, including contacts with the Vatican. The young Roman politician always supported his mentor against attacks from left-wing parties and the Dossetti faction within the DC, until De Gasperi stepped aside in 1953 and died the following year.