A lawyer by training, Sá Carneiro became a member of the National Assembly in 1969 and, in turn, one of the leaders of the “Liberal Wing” which attempted to work for the gradual transformation of António de Oliveira Salazar’s dictatorship into a Western European democracy.

In May 1974, a month after the Carnation Revolution, Sá Carneiro founded the Popular Democratic Party (PPD), together with Francisco Pinto Balsemão, Joaquim Magalhães Mota, Carlos Mota Pinto, João Bosco Mota Amaral, Alberto João Jardim, António Barbosa de Melo and António Marques Mendes, and became its secretary-general. The PPD was soon renamed the Social Democratic Party (PSD); despite Sá Carneiro’s original claims to be leading a left-of-centre party, he and the party soon drifted to the right, becoming the country’s main centre-right force. He was minister without portfolio in a number of provisional governments, and was elected as a deputy to the Constitutional Assembly the next year.

In 1976, he was elected to the Assembly of the Republic. In November 1977, he resigned his office as president of the party, only to be reelected to that office the next year.

In the general election of late 1979, he led the Democratic Alliance, a coalition of his Social Democratic Party, the right-wing Democratic and Social Centre Party, and two smaller parties, to victory. The Alliance polled 45.2 percent of the popular vote and gained 128 of the 250 seats in the Assembly of the Republic; 75 of these were from the PSD. President António Ramalho Eanes subsequently called on him to form a government on 3 January 1980, and formed Portugal’s first majority government since the Carnation Revolution of 1974. In a second general election held in October that year, the Democratic Alliance increased its majority. The Alliance received 47.2 percent of the popular vote and 134 seats, 82 of them from the PSD. Sá Carneiro’s triumph appeared to augur well for the presidential election two months later, in which Sá Carneiro was supporting António Soares Carneiro (no relation).

His victory was short-lived, however. On 4 December 1980, while on his way to a presidential election rally in Porto, the Cessna 421 he was on crashed into a building in Camarate, Loures, soon after takeoff from Lisbon Airport. Eyewitnesses claimed they saw pieces falling from the plane just moments after it took off. Rumours have continued to fuel conspiracy theories that the crash was in fact an assassination, but no firm evidence has come to light. There were even different theories as to whom would have been the target of such an assassination, as Francisco de Sá Carneiro was travelling with the Defence Minister, Adelino Amaro da Costa, who had said he had documents relating to the October surprise conspiracy theory and was planning on taking them to the United Nations General Assembly.