While Adolf Frederick was bishop of Lübeck (1727–50), he administered Holstein-Kiel (1739–45) during the minority of Duke Charles Peter Ulrich (afterward Peter III of Russia). In 1743 he was elected heir to the throne of Sweden by the “Hat” faction. His election was secured by the Russian empress Elizabeth, who, as a result of Russia's defeat of Sweden in the 1741–43 war, was able to demand that Adolf Frederick be named heir under threat of Russian annexation of Finland and the eastern part of Sweden. He thus succeeded to the throne after the death of Frederick I (1751).

Most of the power during the new king's reign rested in the Riksdag (parliament). Twice he tried to free himself of its control. In his first attempt (1756)—aided by his influential wife, Queen Louisa Ulrika, who was sister to Frederick II of Prussia—he nearly lost his throne, but in his second (1768–69)—with the assistance of his son, Crown Prince Gustav—he brought about the overthrow of the generally pro-Russian and pro-Prussian “Cap” Party in the Riksdag. The victorious Hats, however, reneged on their promise to increase the king's power.