Coming from a family of the Bremian Ministerialis at Uthlede, he was a canon of Bremen Cathedral and a clerk of Duke Henry the Lion of Saxony, House of Guelph, before becoming prince-archbishop in 1185.
When the Bremian cathedral chapter elected him for prince-archbishop, due to the competitive politics within Kingdom of Germany at the time, this was regarded a Guelphic triumph.
[1] A canon named Meinhard, originally from the Augustinian monastery at Segeberg (in Hartwig's diocese), was active at Üxküll among the pagan Livonians, apparently attempting to gain converts through preaching.
In 1186, one year into Hartwig's episcopate, the prince-archbishop intervened and gave him the status of a bishop, in effect seizing control of missionary efforts there.
[2] The historian Eric Christiansen judged this to be part of Hartwig's attempt to resurrect his see's former glory, when it "had exercised authority over the entire Northern world".
Hartwig prepared the subjection of the trans-Elbian free peasants republic of Ditmarsh, religiously belonging to the Archdiocese of Bremen but rejecting Bremian secular princely overlordship.
He persuaded Adolphus III of Schauenburg, Count of Holstein, to waive his claim to Ditmarsh in return for regular dues levied from them to be subjected Ditmarsians.
Arriving at Bremen in 1190, Henry VI withdrew Hartwig's princely competences (regalia) and forced him to England and then Lüneburg in exile, returning after a few years.
In 1192 the Bremian Chapter didn't wait any longer for a papal dismissal of Hartwig and unauthorisedly elected Bishop Valdemar as its new prince-archbishop – encouraged by Henry VI.
In the same year Hartwig had convinced Pope Celestine III to confer the same spiritual benefits gained by Levantine crusaders also to Catholics visiting the valley of the Western Dvina.
[8] Together they recruited another large army of Saxon knights, eventually sending an expedition of 500 armed "pilgrims" in 13 naval vessels.