Bosman was born in Utrecht. Although he sailed to the Gold Coast as an apprentice when he was only 16 years of age, he managed to climb the ranks and eventually became head merchant (Dutch: opperkoopman). In 1702 he sailed back to the Dutch Republic, and little is known of his later life.
Bosman is best known for his description of the Gold Coast titled Nauwkeurige beschrijving van de Guinese Goud- Tand- en Slavekust (1703). This document remained the most authoritative description of the area for more than a century and provided significant detail of the Komenda Wars in which Bosman took part. The Ghanaian surname Bossman is thought to originate from the children Willem Bosman had with his black mistresses.