Severin was the first child of the later city Vogts Sæby, Søren Nielsen (around 1655-1730 ), born and his wife Birgitte Ottesdatter. In 1713 he married the 40 years old Maren Nielsdatter (around 1651-1734 ), the wealthy widow of a Copenhagen businessman. With great diligence Severin built in the following years to a thriving company that specialized in the expanding trade with Iceland and Finnmark.
While King Christian VI. initially wanted to give up and trade mission in Greenland on his accession in 1730, succeeded to the missionary Hans Egede to convince him otherwise. The Danish Greenland policy should henceforth be based on three mutually reinforcing pillars: trade of the products of the colony, Christian missionary work and assertion of Danish sovereignty claim.
The proposals met with the Danish merchants but with little enthusiasm, which in the 1720s was the failure of the first trading company, Det mountains Grønlandske Kompagni before eyes. The very experienced in the North Country business Severin took up the idea of the king. 1733 the trade monopoly, he was awarded for Greenland goods, which was renewed in 1740. In addition to grants for the mission Severin was given the right to lead the Danish flag and war weapons. In armed conflicts at sea 1738/39 against the Dutch he put the interests of Danish in Greenland. Severin founded major trade points in Greenland, including 1734 Christianshåb ( Qasigiannguit ) and in 1741 was named after him Jacobshavn (Ilulissat ).
1749 Severin gave back his trade monopoly to the King, who passed it on to a newly established company, from 1774 the Kongelige Grønlandske Trade ( KGH ) was born. She dominated the economic life of Greenland to 1979 completely, was in the 18th century, however, far less successful than Severin. This focused henceforth on the trade in Norway. Through one of his best friends, the missionary Paul Egede, he remained Greenland and the local missionary work life connected.
Jacob Severin became one of the most respected, influential and wealthiest merchants of Copenhagen. He married twice more, in 1735 Birgitte Sophie Nygaard ( 1704-1739 ) and in 1742 his relative Maren Dalager ( 1719-1753 ), who bore him three daughters, and died a few days before him in the headquarters Dronninglund Castle in Aalborg.