Son of Lt-General John Wynyard (1682-1751/2) and Mary Maxwell (ca. 1709-1776). Married (1752) to Mary Sophia Otway (1724-1757), with whom he had one daughter. Married (1758) to Catherine St. Leger (1736-1764). He had at least 4 children with Catherine St. Leger, one of whom rose to the rank of General. Married (1768) to Sarah Lilley (b. 1750), with whom he had 6 children.
Wynyard was educated at Westminister School before, in 1748, he joined the army as an ensign in the 17th Foot. At his death, he had reached the rank of Lt-General and his widow was granted a payment of £200 by King George III in 1790, "for her husband’s "long and faithful services in Our Army."."
It is unclear if Boswell actually met Lt-Col Wynyard, but he certainly met his mother (only identified as "Mrs. Wynyard who has a son a colonel [in the guards]") at a dinner at the Gould's on December 3, 1762.