Frederick was the eldest surviving son of the landgrave of Hesse-Kassel. He fought bravely for England during the War of the Spanish Succession (1701–14). His marriage in 1700 to Louise of Prussia ended with her death in 1705, and in 1715 he married Ulrika Eleonora, sister of Charles XII of Sweden, after first proposing in 1708. She succeeded to the Swedish throne at the death of Charles XII in 1718, but, devoted to Frederick, she abdicated in his favour in 1720, and he was elected king as Frederick I with the aid of anti-absolutist parliamentary forces that had earlier been instrumental in the election of Ulrika, who then inaugurated the Age of Freedom.

During his 31-year reign, Frederick I was virtually powerless, devoting his time to hunting and love affairs. He had several children by his mistress Hedvig Taube, his marriage with Ulrika being childless.