Frederick William was born in Berlin, the son of Prince Augustus William of Prussia (the second son of King Frederick William I of Prussia) and Duchess Luise of Brunswick-Wolfenbüttel. His mother's elder sister, Elisabeth, was the wife of Augustus William's brother King Frederick II ("Frederick the Great"). Frederick William became heir-presumptive to the throne of Prussia on his father's death in 1758, since Frederick II had no children. The boy was of an easy-going and pleasure-loving disposition, averse to sustained effort of any kind, and sensual by nature.
His marriage with Elisabeth Christine of Brunswick-Wolfenbüttel, Crown Princess of Prussia, daughter of Charles I, Duke of Brunswick-Lüneburg, contracted 14 July 1765 in Charlottenburg, was dissolved in 1769. He then married Frederika Louisa of Hesse-Darmstadt, daughter of Ludwig IX, Landgrave of Hesse-Darmstadt on 14 July 1769 also in Charlottenburg. Although he had seven children by his second wife, he had an ongoing relationship with his mistress, Wilhelmine Enke (created Countess Wilhelmine von Lichtenau in 1796), a woman of strong intellect and much ambition, and had five children by her—the first when she was still in her teens.
Frederick William, before the corpulence of his middle age, was a man of singularly handsome presence, not without mental qualities of a high order; he was devoted to the arts — Beethoven and Mozart enjoyed his patronage, and his private orchestra had a Europe-wide reputation. He also was a talented cellist. But an artistic temperament was hardly what was required of a king of Prussia on the eve of the French Revolution, and Frederick the Great, who had employed him in various services (notably in an abortive confidential mission to the court of Russia in 1780), openly expressed his misgivings as to the character of the prince and his surroundings. For his part, Frederick William, who had never been properly introduced to diplomacy and the business of rulership, resented his uncle for not taking him seriously.