Already a committed republican from his youth, well before the 1910 overthrow of the monarchy, he became a member of the National Constituent Assembly, elected on 28 May 1911.

He gave up his constituency in order to join the army; during Portuguese participation in World War I, he saw combat himself, and upon returning home he wrote a book about his battle experiences.

Afterwards he was nominated Prime Minister again, to take the place of another liberal, Tomé de Barros Queirós, on 30 August 1921.

During the infamous "Bloody Night" in Lisbon, on 19 October 1921, Granjo was assassinated.

That same night, two other prominent republicans of moderately right-wing sympathies, António Machado Santos (widely known as the founder of the republic) and José Carlos da Maia, also died.