Hallgrímur Pétursson was likely born at Gröf in Skagafjörður. He grew up at Hólar, where his father was the bell-ringer and his cousin Guðbrandur Þorláksson the resident bishop. As a young man, he left Hólar for unknown reasons and travelled to mainland Europe, possibly to learn the blacksmith trade. He ended in Copenhagen, where Brynjólfur Sveinsson sponsored him to attend the seminary at the Church of Our Lady. Brynjólfur had family connections to Hallgrímur, as his half-brother was married to Hallgrímur's aunt.