At the outbreak of the Revolutionary War, he was fully 53 years of age, but being experienced in soldiering, he served as Captain of the Frederick County Militia. It has also been said that he was the commander of the barracks at Frederick Towne. At one time during the War it was ordered that Captain William Duvall be appointed collector of the fines in Linganore Hundred in place of Abraham Haaff. (MD. Hist. Magazine, vol. 12, p. 19) 
His land transactions in Frederick County were numerous. The first seems to be 1762, when as William Duvall of Benjamin he purchased from Henry Gordon, of Frederick Co., the tract know as "Gordon's Chance". On February 14, 1769, he bought of Charles Hammond Jr., son and heir of Philip Hammond, late of Anne Arundel County, three tracts, one being "Benjamin's First Choice'", lying on Bennett's Creek, which had been granted to Benjamin Ryan, but since October 1754 had been in the "use" of Philip Hammond. Other tracts held by him were "Duvall's Forrest", "Good Friday", "May's Fall", and "Battis Fonts."
Despite being a large land owner, he was not a slave owner according to the 1790 census. In his household in that year were himself and another male over 16, one male under 16, and two females.
According to family records cited in Newman, his wife died in 1798, but he survived until 1810. No definitive sources are available.
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