Son of Duke Bogislaw II and Miroslava of Pomerelia, he succeeded to the Duchy of Pomerania-Stettin upon his father's death in 1220;[1] he had, however, to share the rule of Pomerania with his cousin Wartislaw III, who resided at Demmin.

At first still a Danish fief, the Pomeranian lands fell back to the Holy Roman Empire after the victory of several North-German princes at the 1227 Battle of Bornhöved.

Emperor Frederick II of Hohenstaufen in 1231 put the Duchy of Pomerania under the suzerainty of the Ascanian margraves of Brandenburg, disregarding the tenure of the Griffin dynasty, and thereby fueling the long-term Brandenburg–Pomeranian conflict.

After his cousin Wartislaw III had formally accepted the Brandenburg overlordship by the 1236 Treaty of Kremmen, Duke Barnim I came to terms with the mighty Ascanian margraves in the 1250 Treaty of Landin: he confessed himself a Brandenburg vassal and had to renounce the Uckermark region, nevertheless he reached the consent, that the fief of his cousin Wartislaw would remain with the Griffin dynasty upon his death.

He promoted the Ostsiedlung by introducing German settlers and customs into the duchy, established many towns, among them Prenzlau, Szczecin, Gartz, Anklam, Stargard, Gryfino, Police, Pyrzyce, Ueckermünde and Goleniów.