Dorothea Maria Lösch was the daughter of the goldsmith Henrik Jakob Lösch from Stockholm and Dorothea Maria Beyms and married in 1756 to the Finnish sea captain Mårten Johan Thesleff: her spouse's name was also spelled Theslöf or Theslef.
She was the author of a medical book of how to treat smallpox, Beskrivning af et bepröfvat medel emot Kopp-ärr (Stockholm, 1765).
Dorothea Maria Lösch took over and commanded the ship Armida to safety after its officers had been killed or abandoned it during the Battle of Svensksund on 9 July 1790.
For this act, she was awarded with the rank of a master mariner of the Swedish fleet, something unique for a woman of this period.
Although this was a purely honorary, ceremonial title, she was nevertheless the first woman in such a position.