He graduated in law from University of Coimbra in 1959.
He was Minister of Social Affairs in the II Constitutional Portuguese Government, led by Mário Soares. In 1973, together with personalities like Mário Soares or Salgado Zenha, he founded Socialist Party, in Bad Münstereifel, Germany. In 1975 he was elected to the Constituent Assembly, which had the task of drafting the new Constitution after the Carnation Revolution.
He was also elected to the Assembly of the Republic for several times.
In 1978 he was sworn in as Minister of Social Affairs, and despite being in office for only seven months, he founded the Portuguese National Health Service, which created the first universal health system in Portugal. Although being retired from active politics, António Arnault is still an influential voice in the country.
He was President of the Portuguese Bar Association council in Coimbra District. He is a freemason, and was Grand Master of Grande Oriente Lusitano between 2002 and 2005.
He was a member of the party board until 1983.