Son of Prince August William (1722-1758) of Prussia, and Louise Amalie (1722-1780) of Brunswick-Lüneburg. Nephew of Frederick II (1712-1786) the Great, whom he succeeded as King of Prussia in 1786. He married firstly Elisabeth Christine of Brunswick-Lüneburg, with whom he had one child, Princess Frederica Charlotte (1767-1820). They divorced in 1769 and shortly after Frederick married Frederika Louisa of Hesse-Darmstadt (1751-1805), with whom he had seven children. He had at least twelve further children with at least three different women.
He is mostly known today for having mismanaged the state of Prussia to an extent so that the economic and military power which Frederick II left him in 1786, had gone bankrupt and lost most of its military power by the time of his death in 1797.
Boswell was introduced to the future King by George Keith in Potsdam on July 3, 1764.