In late 1739, he became garrison commander at Fort William, succeeding Captain Buncombe. In the 1735, Randall bought his commission and served as a captain of dragoons. His elder brother, Edward Randall, inherited the family estate in Sussex, while his younger brother, Alexander, became a curate. His father was knighted in the late 17th century, and was awarded a baronetcy by King George I.
Jonathan was the second son of Sir Denys Randall and Jessica Wolverton.
