Frederick William was born in Berlin to George William, Elector of Brandenburg, and Elizabeth Charlotte of the Palatinate. His inheritance consisted of the Margraviate of Brandenburg, the Duchy of Cleves, the County of Mark, and the Duchy of Prussia.
During Thirty Years' War, George William had striven to maintain with a minimal army a delicate balance between the Protestant and Catholic forces fighting throughout the Holy Roman Empire. Out of these meagre beginnings Frederick William managed to rebuild his war-ravaged territories. In contrast to the religious disputes in other European states, the elector supported religious tolerance. With the help of French subsidies, he built up an army to defend the country. Through the treaties of Wehlau, Labiau, and Oliva, Frederick William succeeded in revoking Polish sovereignty over the Duchy of Prussia, leaving the Holy Roman Emperor as his only liege.
Frederick William was a military commander of wide renown; his standing army would later become the model for the Prussian Army. He is notable for his joint victory with Swedish forces at the Battle of Warsaw (1656), but the Swedes turned on him at the behest of King Louis XIV of France and invaded Brandenburg. After marching 250 kilometers in 15 days back to Brandenburg, he caught the Swedes by surprise and managed to defeat them on the field at the Battle of Fehrbellin, destroying the myth of Swedish military invincibility. He later destroyed another Swedish army that invaded the Duchy of Prussia during the Great Sleigh Drive in 1678. He is noted for his use of broad directives and delegation of decision-making to his commanders, which would later become the basis for the German doctrine of Auftragstaktik, and he is noted for using rapid mobility to defeat his foes.
Frederick William is notable for raising an army of 40,000 soldiers by 1678, through the General War Commissariat presided over by Joachim Friedrich von Blumenthal. He was an advocate of mercantilism, monopolies, subsidies, tariffs, and internal improvements. Following Louis XIV's revocation of the Edict of Nantes, Frederick William encouraged skilled French and Walloon Huguenots to emigrate to Brandenburg-Prussia with the Edict of Potsdam, bolstering the country's technical and industrial base. On Blumenthal's advice he agreed to exempt the nobility from taxes and in return they agreed to dissolve the Estates-General. He also simplified travel in Brandenburg and the Duchy of Prussia by connecting riverways with canals, a system that was expanded by later Prussian architects, such as Georg Steenke; the system is still in use today.
On 7 December 1646 at The Hague, Frederick William entered into marriage, proposed by Blumenthal as a partial solution to the Jülich-Berg question, with Luise Henriette of Nassau (1627-1667), daughter of Frederick Henry of Orange-Nassau and Amalia of Solms-Braunfels. Their children were
On 13 June 1668 at Gröningen, Frederick William married Sophie Dorothea of Holstein-Sonderburg-Glücksburg, daughter of Philipp of Holstein-Sonderburg-Glücksburg and Sophie Hedwig of Saxe-Lauenburg. Their children were