As the second son of King Afonso II of Portugal, he was not expected to inherit the throne, which was destined to go to his elder brother Sancho.

Until his brother's death and his own eventual coronation, Afonso retained and used the title of Visitador, Curador e Defensor do Reino (Overseer, Curator and Defender of the Kingdom).

[6] Determined not to make the same mistakes as his brother, Afonso III paid special attention to what the middle class, composed of merchants and small land owners, had to say.

In 1254, in the city of Leiria, he held the first session of the Cortes, a general assembly comprising the nobility, the middle class and representatives of all municipalities.

Progressive measures taken during his kingship include: representatives of the commons, besides the nobility and clergy, were involved in governance; the end of preventive arrests such that henceforward all arrests had to be first presented to a judge to determine the detention measure; and fiscal innovation, such as negotiating extraordinary taxes with the mercantile classes and direct taxation of the Church, rather than debasement of the coinage.