When Duke Albert of Brunswick died in 1279, his lands were divided amongst his three sons, the eldest becoming Duke of Brunswick-Grubenhagen. The lands of the Duchy lay south of the Harz in two districts. The more westerly was a small district around the town of Einbeck and included the castle of Grubenhagen; the other was larger and included the town of Osterode. In the 11th century the region had belonged to the County of Katlenburg, which was near Osterode.
The Duchy was partitioned in 1322 into Grubenhagen and Osterode, and the latter in 1361 into Salzderhelden (near Grubenhagen) and Osterode. The Duke of Brunswick-Salzderhelden administered Grubenhagen when its Duke went off to become the fourth husband of Queen Joanna I of Naples in 1376 and inherited it when he died in 1399.
Grubenhagen was reunited under the Osterode line in 1526. Never large, Grubenhagen was by then even smaller. Einbeck had long since passed to the Göttingen branch, whilst the northern part of the Eichsfeld, the southern lands of the district around Osterode, were pawned in three stages to the Archbishops of Mainz and never redeemed. In 1596 the last of the line died and the Duchy was transferred to the Wolfenbüttel line, though in 1617 the Dukes gave it up to Brunswick-Celle. In 1665 it was added to Brunswick-Calenberg.