[2] Together with Just Fabritius he was a driving force behind the establishment of the short-lived Danish Africa Company and served as its director from 1755 to 1767.
Hemert was one of several merchants Johann Friedrich Struensee consulted in 1770 in connection with his work for more liberal conditions for the business community.
He then married, for a second time, Petronelle Elisabeth Mestecker (13 September 1703 – 19 February 1778) on 14 October 1732 in Hamburg.
Joost van Hemert owned a large house in Købmagergade (present day No.
44) as well as the country house Frederiksdal Lyststed (now Tusculum) on the north side of Bagsværd Lake.