Niezer was born in Akwida as the son of a German doctor's assistant in the service of the Dutch West India Company, and an African woman named Manzang.
Around 1770, Jan Niezer returned to the Gold Coast alone, joining the Dutch West India Company, like his father.
When the Dutch East India Company was dissolved in 1791, Niezer took his chance to become the biggest private trader on the coast.
1794 was also the beginning of a twenty-year episode in the history of the Dutch Gold Coast, in which the colony was increasingly severed from its home country due to wars in Europe.
Negotiations between the parties failed, however, leaving Niezer with no option but to release Apho to the mob, which subsequently killed him.
As the Ashanti had won the Battle of Abora, Niezer was confident that the western trade route would soon be open again, and left for Elmina.
Jan Niezer supplied the Elminese with weapons and munition worth 11,000 Dutch guilders, and led a battle group of around 200 men against the aggressors.
His sons had in the meantime sold much of his property and spent most of his wealth, so that Niezer in 1821 was unable to pay the debt he owed in Cape Coast.