Son of Duke Bogislaw II and Miroslawa, daughter of Duke Mestwin I of Pomerelia, he succeeded to the Duchy of Pomerania-Stettin upon his father"s death in 1220. He had however to share the rule of Pomerania with his cousin Wartislaw III, who resided at Demmin. A minor when his father died, his lands until about 1226 were under regency of his mother Miroslawa from the Pomerelian Samborides dynasty.
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 Hohenstufen 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. When Duke Wartislaw III died in 1264, Barnim I was able to unite the whole Duchy of Pomerania under his rule.
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 GolenióWest
He was also known for his generous ecclesiastical foundations, and supported the extension of the secular reign of the Cammin bishops in the Kołobrzeg area. Duke Barnim died at the town of Dąbie (Altdamm), today part of Szczecin.
The Minnesinger Meister Rumelant wrote a dirge in his honour.
