Son of Francis van Aerssen van Sommelsdyck (1660-1740), Admiral, a first cousin of Boswell's grandfather, James, and Maria van Aerssen van Wernhout. Married to Everdina Petronella van Hogendorp (1730-1794), Countess of Hogendorp.
In the 17th century, the Sommelsdyck family was one of the richest families in the country, owning a third of the Dutch possessions in Surinam. They were still quite well off at the time of Boswell's visit.
Boswell met Herr van Sommelsdyck at the Hague on December 24, 1763. Their meeting seems to have been coincidental, although Boswell had planned to visit Sommelsdyck. He was immediately invited to dine with the family, which he did on the next day.