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.
Van Aerssen was Lord (Vrijheer) of Sommelsdijk, Plaat, Spijk, and Ooltgensplaat. He pursued a military career in the Dutch States Army, reaching the rank of Colonel in the infantry and later Major General. He resided primarily in The Hague.
Through his father, he was a second cousin to James Boswell (their fathers were first cousins), as Boswell's grandmother, Veronica van Aerssen van Sommelsdyck, was the sister of Francis's grandfather. Boswell often referred to the family as his "Dutch relations," benefiting greatly from their hospitality and high status in Dutch society.
In the 17th century, the Sommelsdyck family was one of the wealthiest in the country, owning one-third of the Society of Suriname (which administered the colony). While the family was still affluent during Boswell's visit, it was this Francis who eventually sold the family's share of Suriname to the City of Amsterdam in 1770 for 700,000 guilders, ending the family's nearly century-long influence in the South American colony.
Boswell was introduced to Herr van Sommelsdyck at the Hague on December 23, 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 December 24.