Their maternal grandparents were Leopold I, Holy Roman Emperor, and Eleonor Magdalene of Neuburg, sister of Queen Maria Sofia of Portugal.

Peter married his niece Maria, Princess of Brazil, in 1760, at which time she was the heiress presumptive to the throne then held by his brother Joseph I.

Peter made no attempt to participate in government affairs, spending his time hunting or in religious exercises.

He also defended the high nobility of Portugal, and sponsored the petitions of those accused in Távora affair, whose rehabilitation was subject of new lawsuits, in which the heirs demanded the restitution of their confiscated properties.

Peter III was moderately friendly toward the Jesuits, who had been banished from Portugal and its overseas empire in 1759, largely at the behest of the Marquis of Pombal.