Arnaut, a lawyer, was born in Cumeeira, Penela, Coimbra district, on 28 January, 1936, and had recently been in the hospitals of the University of Coimbra.

Honorary President of the PS since 2016, Arnaut was Minister of Social Affairs in the 2nd Constitutional Government, Grand Master of the Lusitanian Grand Orient and was awarded the rank of Grand Officer of the Order of Liberty and the Grand Cross of the Order of Liberty.

Arnaut, who was also a poet and writer, from a young age was involved in the resistance against the Estado Novo dictatorship of António Salazar, and participated in the district committee of Coimbra of the presidential candidacy of Humberto Delgado, who stood against Salazar's candidate in the presidential elections of 1958.