Lorentz Johannsen was born at Glücksburg in the district of Schleswig-Flensburg in Schleswig-Holstein (now in Germany). In 1801, he was married with Magdalena Boletta Knudtzon (1779-1803). In 1804, he married Sarah Marie Knudtzon 1782-1854), sister of his first wife.
Both of his wives were daughters of Hans Carl Knudtzon (1751-1823) who was a successful merchant in Trondheim, Norway. Hans Carl Knudtzon had been born in Bredstedt, in the district of Nordfriesland, Schleswig-Holstein. Knudtzon had moved to Trondheim where he founded Hans Knudtzon & Co. a wholesale trading company. Following his marriage, Lorentz Johannsen became a partner with his father-in-law at the trading firm. The company grew to become one of foremost trading firms in Trondheim. The firm operated its own fleet of ships and was engaged in the trade of commodities. [2][3]