He served as Prime Minister of Portugal for eleven months during 1980, until his death in a plane crash in Camarate on 4 December 1980.

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.

Rumours have continued to fuel conspiracy theories that the crash was in fact an assassination, but no firm evidence has come to light.

[5] Dependent to a considerable extent on Sá Carneiro's personal popularity, the Democratic Alliance was unable to maintain its momentum in the wake of his death.

Faced with a national crisis, the public rallied behind the incumbent president, António Ramalho Eanes, who easily defeated the Alliance candidate in the presidential election a few days later.

Sá Carneiro started his political life in the youth of the Acção Católica (the Portuguese Catholic Action), being his first activity in civic life to write a letter to Marcelo Caetano requesting the return of the António Ferreira Gomes, the exiled pro-democracy bishop of Oporto.

Sá Carneiro tried to adapt the social-democratic ideas of the likes of Eduard Bernstein and the post-1945 SPD to the cultural context of Portugal[8] and its traditionally Catholic society.