His mother died when William was a boy, and his father remarried to Mary Burbridge, who gave birth to Robert Burbridge Armistead, his half-brother (Robert B. Armistead would also serve in the Virginia House of Delegates during the 1796 term and named a son "William", who would marry a granddaughter of once powerful speaker John Robinson).
As was the custom at the time, his father informally gave him an enslaved boy as a child, who was raised with him and would become his personal manservant.
The fire which burned down both the clerk's office and nearby jail had been part of an escape attempt of John Price Posey, who had been steward of the Pamunkey River plantation for the underage George Washington Parke Custis.
[2] Despite the fire losses, New Kent County voters elected Armistead as one of their representatives in the House of Delegates in 1784, and re-elected him once.
During the 1787 Virginia tax census, William Armistead continued to own 12 enslaved adults and nine younger slaves, as well as three horses and 26 cattle, although only his half-brother Robert lived in the county.