Daughter of Francis van Aerssen van Sommelsdyck (1660-1740), Admiral, a first cousin of Boswell's grandfather, James, and Maria van Aerssen van Wernhout. Sister of Francis Cornelius van Eersen van Sommelsdyck. Married (1730) to Philippe de Jouccourt (b. ca. 1709). Married (1742) to Aarnoud Joost van der Duyn.
Boswell met Anna Margaretha on December 28, 1763, possibly at Baron Sporck's in The Hague. In a memo, he wrote that Madame Maasdam told you how la Comtesse was taken to Utrecht, as there were strange suspicions. "She is a little galante, but do not whisper it".1