Johann Jakob was born in Bergnassau, one of three sons of the administrator Simon Moritz Bethmann (1687–1725).
After the premature death of their father, the brothers were raised by their maternal uncle Jakob Adami in Frankfurt am Main.
Besides trading in goods and merchandise, he was not averse to an engagement in the slave trade and had already secured a guaranty worth 300,000 livres from his brothers in Frankfurt for that purpose before the plan for a joint venture with the Danish West India and Guinea Company was aborted due to business uncertainty.
[2] He left his daughter Katharina Elisabeth (1753–1813) in the care of his childless brother Simon Moritz in Frankfurt.
Johann Jakob Bethmann was a friend of German author Friedrich Melchior Grimm.