 Hemert was born in Copenhagen on 20 May 1734, the son of merchant and shipowner Joost van Hemert (1696–1775) and Petronelle Elisabeth Behagen, née Mestecker (1703–78).His mother was the widow of Anthony Behagen (1687–1727)), a wine merchant from Hamburg, and had brought a son, Gysbert Behagen, into the marriage.Peter van Hemert was educated in his father's company.