Thormøhlen was born c. 1640 in the Duchy of Holstein, at that time a hereditary possession of the King of Denmark as Duke of Holstein-Glückstadt.
[2] Thormøhlen was the main owner of the Norwegian slave ship Cornelia, which in 1673 sailed to West Africa transporting a cargo of liquor and trinkets which it exchanged for a number of enslaved Africans; 103 slaves were transported to the island of Saint Thomas in the Danish West Indies by the Cornelia.
In the next year, the Cornelia made another triangular trade voyage, again delivering 103 slaves to Saint Thomas.
From 1685 to 1693, Thormøhlen owned the Kronstad Hovedgård manor house, which was located in the borough of Årstad (on the southern shore of the bay of Store Lungegårdsvannet) south of the city.
However, heavy financial losses in overseas trade along with a major series of structure fires at Bryggen in 1702 created difficulties for Thormøhlen, who eventually went bankrupt.