Serving in the South Carolina Militia, rising to be commissioned as brigadier general in the Continental Army in the American War of Independence.
He led numerous campaigns against Loyalists and Cherokee, who in 1776 had launched an attack against frontier settlements across a front from Tennessee to central South Carolina.
Following the fall of Charleston to the British in 1780 after a month-long siege, and the capture of thousands of American troops, the Patriot resistance was effectively subdued in South Carolina and Georgia.
After the war, Patriot General Nathanael Greene testified that Williamson had acted in Charleston to collect intelligence and pass it to the Americans; he was the "first major double agent" in America.
He bought a plantation, called Hard Labor, near a creek by the same name, and in Greenwood County near the settlement of Ninety Six.
[2] In 1760, he was commissioned as lieutenant in the colonial militia, a step up to the officer corps for a man most sources agree was likely illiterate.
[2] He became highly influential by the time of the Revolution, having been promoted to the rank of major, established continuing relations with Henry Laurens and other colonial leaders, and served on provincial commissions.
In 1774 and 1775, he was elected to South Carolina's Provincial Congress with other men from Ninety-Six District, including Francis Salvador and Richard Rapley.
[2] At the start of the American Revolutionary War, Williamson fortified his plantation, which he later used as his military headquarters, and as a fort, prison and arms depot.
[2] (The British added to the star fort in the settlement, as they considered this an important strategic location and wanted to control it.)
[2] From the Patriots' view, Williamson's campaigns against the Cherokee were a success; they petitioned for peace and ceded more than a million acres of land in what is now the counties of Anderson, Pickens, Oconee and Greenville in South Carolina.
He was well liked as a commander and the Cherokee called him “Warrior Beloved Man.”[2] Charleston fell in 1780 after a month-long siege and 5,000 American troops were captured.
The British were trying to dismantle the opposition by offering the rebels parole and protection, if they promised not to take arms against the Crown again, nor incite others to do so.
Historian Llewellyn M. Toulmin's conclusion based on the documentation and Williamson's actions is that he had taken protection and was assisting the British, including supplying them.
Some later returned to the Rebel side, angered as the British tried to have senior officers rally younger men against the American cause.
[2] After the war, the South Carolina General Assembly voted to confiscate Williamson's White Hall plantation.
(White Hall eventually disappeared and has been the object of a to-date unsuccessful search by descendants, state and Federal archaeologists.)
[4][5] Historian Llewellyn M. Toulmin has written of Williamson, "Because of his high rank and important information that he passed on for almost a year, he can fairly be described as 'America’s first major double agent.