Oliveira Marques was born in the 'freguesia' of S. João do Estoril, Cascais. He studied history and philosophy and graduated in 1956 from the University of Lisbon with a thesis entitled A Sociedade em Portugal nos Séculos XII a XIV (Portuguese Society from 12th to 14th Centuries). After training in the University of Würzburg, in Germany, he would start teaching the following year at the University of Lisbon and gain a PhD in 1960. His PhD dissertation was entitled Hansa e Portugal na Idade Média (Hanseatic League and Portugal in the Middle Ages).

In 1962 his participation in student strikes against the right wing regime of Oliveira Salazar resulted in his dismissal from the university as a scholar. He left for the United States where he taught history at a number of universities (Auburn, Florida, Columbia, Minnesota and Chicago) between 1965 and 1970.

Oliveira Marques returned to Portugal in 1970, but only resumed teaching at Portuguese universities in 1974, the year in which the Carnation Revolution ended almost five decades of the Estado Novo regime. From October 1974 to April 1976 he was director of the National Library of Lisbon.

In 1998 he was granted the Grand Cross of the Order of Liberty, a state accolade which distinguishes figures who have made a contribution to the values of civilization.

Considered one of Portugal's greatest experts on Portuguese Medieval history, he wrote more than 60 books and numerous articles. One of his most famous works, A History of Portugal, was translated into several languages.