As a surveyor, Buchanan was able to locate and purchase some of the most desirable plots of land in western Virginia and quickly became wealthy and politically influential.

Several sources say he was the son of James Buchanan and Jane Sayers, and that the family lived for a time in Northumberland County, Pennsylvania.

[12]: 43 In December 1742, the militia engaged in combat with a group of twenty-two Onondaga and seven Oneida Indians who had traveled to Virginia from Shamokin in Pennsylvania, under the command of an Iroquois chief named Jonnhaty, to participate in a campaign against the Catawba.

[13]: 643–646  The grandson claimed that suspicious white settlers, thinking that this war party planned to raid Virginia settlements, attacked them.

The militia followed the warriors for two days, until one of the Indians made a detour into the forest near Balcony Falls, possibly to relieve himself, and a militiaman fired at him.

The next spring he moved to Cherry Tree Bottom Plantation at Looney's Ferry on the James River, where he was visited by Colonel George Washington in October.

[1]: 368–371 In February-April 1756, Buchanan led a company of rangers on the Sandy Creek Expedition, intended to assault the Shawnee village known as Lower Shawneetown, from which raids on Virginia settlements had been launched.

[20] On 27 July 1756, Buchanan presided over a council of war, held at the Augusta County Courthouse, "to meet and consult on the most proper places to build forts along the fronteers for the protection of the Inhabitants."

Present were Colonel David Stewart, Major John Brown, and ten captains, all officers of the Augusta County militia.

[22] On 23 August 1756, Governor Dinwiddie wrote Peter Hog: “I have recommended Colo. Buchanan to him [Clement Read] for Augusta Coty.

I have a bad Opinion of Colo. Stewarts Conduct, & before he receives any Mony, I shall make a Strict Scrutiny into his Demands, & think it must go through Buchanan's Hands.

[27]: 222 Buchanan served as an Orange County magistrate for the Augusta district from 3 November 1741 to 10 October 1745, at which time the Virginia Council included him as the third most senior magistrate (subordinate only to James Patton and John Lewis) in the first Augusta County commission of the peace [28]: 191, 214 [15]: 84  In 1742 he was appointed tax collector and exterminator of wolves.

[30] Among his 20 company members were John Buchanan, George Robinson, James Wood, Adam Harman, Israel Lorton, and Peter Rentfroe.

[39]: 177–78 [40]: 86 In 1781, Buchanan's career as a surveyor became the focus of a major investigation, as a result of a lawsuit filed by William Ingles over his holdings in Burkes Garden.

In 1756, they moved to Cherry Tree Bottom plantation at Looney's Ferry on the James River, at the mouth of Purgatory Creek, near where the town of Buchanan now stands.

[53]: 200–202 John and Margaret had seven children:[1] On 22 January 1767, Buchanan purchased six slaves at a cost of £337.10 total, as a gift for his daughter Mary.