The Province comprised large parts of the North German Plain, stretching from the Elbe river in the west to beyond the Oder in the east, where the Neumark region bordered on the Prussian Grand Duchy of Posen (Province of Posen from 1848). Other neighbouring provinces were Pomerania in the northeast, Silesia in the southeast, and Prussian Saxony in the southwest. Brandenburg also shared a common border with the grand duchies of Mecklenburg-Schwerin and Mecklenburg-Strelitz in the northwest as well as with Anhalt in the west.
Beside the Elbe and Oder river areas, the province covered large parts of the Spree and Havel basin. The largest cities were Berlin, located in the centre together with the growing suburbs of Spandau, Charlottenburg, Schöneberg and Neukölln. Larger towns were the royal residence Potsdam and the regional capital Frankfurt (Oder), furthermore Landsberg (present-day Gorzów Wielkopolski) in the east, the historic capital Brandenburg an der Havel as well as Cottbus, Forst and Guben (Gubin) in Lower Lusatia.
The first people who are known to have inhabited Brandenburg were the Germanic Suebi. During the Migration Period, they were succeeded by the Polabian Slavs, whose fortress at Brandenburg an der Havel was conquered by the German king Henry the Fowler in 928/29. Henry subdued the Slavic tribes up to the Oder river and his son Otto I established the marca Geronis on their territory, with the government first conferred to the Saxon count Gero.
The Northern March was split off in 965, however, large parts were again lost in the Great Slav Rising of 983, and the margravial title did not become hereditary until the time of Albert the Bear, another Saxon count from the noble House of Ascania, who established the Margraviate of Brandenburg in 1157. His son Margrave Otto I already achieved the dignity of an Arch-Chamberlain of the Holy Roman Empire in 1177. Emperor Charles IV by the Golden Bull of 1356 confirmed the electoral dignity of the Brandenburg margraves and in 1373 assigned the electorate to his son Wenceslaus in 1373. The Elector of Brandenburg held the seventh rank among the electors of the Empire and had five votes in the Council of Princes.
In 1415 Brandenburg was acquired by Burgrave Frederick of Nuremberg, the first member of the Swabian House of Hohenzollern to rule the margraviate. Over the centuries, the Hohenzollerns gradually rose to one of the most important dynasties of the Empire, rivalling with the ruling House of Habsburg, a process that intensified with the Protestant Reformation and the inheritance of the Polish Duchy of Prussia in 1618. The margraviate formed the core of the Brandenburg-Prussian state and the "Great Elector" Frederick William I made various accessions to the territory, the Treaty of Königsberg of 1656 marking a significant turn in its evolution. By the 1657 Treaty of Wehlau, Frederick William reached full sovereignty in his Prussian territories, which enabled his son Frederick I to assume the crown of a "King in Prussia" in 1701.