One of the most divisive figures in modern Portuguese history, he was referred to by the writer Fernando Pessoa as the "President-King", a description that stuck in later years and symbolizes his regime.

[4] As a leading member of the Constituent Assembly, Pais was appointed Minister of Public Works in the government chaired by João Chagas, assuming his office on 24 August 1911.

At a moment when international tensions that would lead to World War I already made themselves felt, Pais was appointed to the post of Minister Plenipotentiary (ambassador) of Portugal in Berlin on 17 August 1912.

[4] He remained in that important diplomatic post during the critical period that led to the outbreak of the war, maintaining a difficult balance between the pressures of the Portuguese Government, with increasingly pro-war and Anglophile viewpoints, attempts to settle diplomatically border conflicts in areas of contact between the Portuguese and German colonies in Africa, and his own increasingly Germanophile position.

Despite these difficulties, he held the position until 9 March 1916, the date on which Germany declared war on Portugal following the seizure of German ships in ports under Portuguese control.

[4] Instead of starting the usual consultation for the formation of a new government, the rebels took power, removing Bernardino Machado from the post of President of the Republic and forcing his exile.

During the coup and the early stages of his government, Sidónio Pais enjoyed the support of various labor groups, in exchange for the release of imprisoned comrades, and because of expectations inside the influential National Workers' Union, which looked to position itself as a centre of power of the republican left.

[4][7] Subsequently, Pais issued a set of dictatorial decrees, without consulting the Congress of the Republic, and suspended important parts of the Constitution, giving the regime a markedly presidential image.

In this new political architecture, which his supporters called a "New Republic", the Head of State was placed in a position of power that had known no parallel in Portuguese history since the end of absolutism.

He was proclaimed President of the Republic on 9 May of the same year, without even bothering to consult Congress, and enjoying direct democratic legitimacy, which he used – unsuccessfully – to crush opposition attempts.

Between alternating strikes, conflicts, and conspiracies, from the summer of 1918 onwards attempts to end the "Sidonist" regime escalated in severity and violence, which led the President to declare a state of emergency on 13 October 1918.

With that act, and the harsh repression of opposing movements, he was able to regain momentary control of the political situation, but his regime was clearly mortally wounded.

Pais escaped a first assassination attempt on 5 December 1918, during the award ceremony for survivors of the navy trawler Augusto de Castilho.

The president made his way to the Rossio railway station in Lisbon on the evening of 14 December 1918, after having enjoyed dinner at the restaurant Silva, located in the Chiado.

Sidónio Pais's funeral was attended by tens of thousands of people, but multiple interruptions, some of them violent, took place as protesters mingled among the crowd.

[3] Particularly among the most conservative Catholic groups, Pais entered the Portuguese imagination as a mix of a savior and a martyr, and caused the emergence of a popular cult, similar to the one existing around the figure of José Tomás de Sousa Martins, which endures until today.

For instance, when the Portuguese National Pantheon was inaugurated in 1966, the authorities had Pais's body transferred to it from the Room of the Chapter of the Jeronimos Monastery, where it had been interred previously.