Born in Cölln on the Spree (today part of Berlin), George William was the son of John Sigismund, Margrave of Brandenburg and Anna of Prussia. His maternal grandfather was Albert Frederick, Duke of Prussia. In 1616, he married Elisabeth Charlotte of the Palatinate. Their only son Frederick William would later be known as the "Great Elector". Of his two daughters, the eldest, Louise Charlotte, married Jacob Kettler, Duke of Courland, and the younger, Hedwig Sophie, married William VI, Landgrave of Hesse-Kassel.