David went to Spain in the 1760s to become a merchant at Valencia, and at that time changed his name to Thomas (David) Boswell. He returned to England in 1783, and in December 1783 he married Anne Catherine Green. The couple had four children, two of whom, Maria and Thomas Alexander, survived infancy. Thomas David and Anne appear to have separated permanently in 1799.
In 1791 he became a clerk in the Navy Pay Office, while still keeping his private business as a merchant and a banker. Over the years, he rose through the ranks in the Pay Office, which enabled to acquire fairly substantial land holdings. Most notably, in 1803 he bought Crawley Grange, a Tudor mansion in Buckinghamshire and reputedly the sometime home of Cardinal Wolsey. Later, he would acquire Astwood, Filiol Manor, and Broughton Manor as well as other estates.
By 1811 he is known to have lived at 45 Upper Norton Street (presently known as Bolsover Street) in London, close to the former London home of his brother James. He died there in May 1826.
David Boswell enters his older brother's journals when he arrived in Edinburgh with their mother on November 10, 1762. The two brothers were rather close, often of the same mind and they corresponded often during the long periods of physical absence from each other.
The most extensive biography of Thomas David Boswell is probably John Stone's article Being Boswell's Brother, which was printed in The Age of Johnson 23 (2015): 205-238.