He served as the ninth president of Portugal (the first of the Military dictatorship) and prime minister for a brief period of time.

[4] As a revolutionary with moderate tendencies, Mendes Cabeçadas thought it possible to form a government that wouldn't question the constitutional regime, but with no influence on the Democratic Party.

However the other revolutionaries (among them Gomes da Costa and Óscar Carmona) judged him as incapable and in a meeting in Sacavém on 17 June 1926, Mendes Cabeçadas was forced to renounce the posts of president of the republic and president of the Council of Ministers (prime minister) in favour of Gomes da Costa.

[3][2] Mendes Cabeçadas joined the opposition to the regime for a third time, involving himself in several revolutionary attempts and subscribed to many manifestos against the dictatorship, until his death in 1965[5] during the period known as the Estado Novo (New State), headed by António de Oliveira Salazar.

[4] Mendes Cabeçadas married Maria das Dores Formosinho Vieira (Silves, 6 January 1880 – 22 December 1949) in Santa Isabel, Lisbon, in March 1911.