Brigadier-General Andrew Williamson (c. 1730–1786) was a Scottish-born trader, planter, and military officer. 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. Williamson was particularly effective in suppressing the Cherokee, killing an unknown number of Cherokees and destroying 31 of their towns. As a result of his Indian campaign, the Cherokee ceded more than a million acres in the Carolinas.

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. Williamson, like some other officers and troops, appeared to sue for peace. While the circumstances were not clear, he appeared to be aiding the British. He was captured by Americans and gained release, moving from White Hall close to Charleston. There he was taken captive in a second American raid, but freed by British forces. 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.

Williamson was most likely born in Scotland. As a child he emigrated with his parents, whose names are not known, to British Colonial America. They settled on the western frontier in Ninety Six, South Carolina, which was called the Long Cane District.

Like many other Scots on the frontier, Williamson became a trader, known by 1758 to be supplying cattle and hogs to frontier forts. He may also have driven cattle to Charles Town to market. He bought a plantation, called Hard Labor, near a creek by the same name, and in Greenwood County near the settlement of Ninety Six. He renamed it as White Hall. He also purchased slaves to work the property. It was one of the few large plantations in this area. Much of the backcountry was being settled by Scots and Scots-Irish subsistence farmers, many of whom were Loyalists in the Revolution. Williamson was contracted to build a fort at Ninety Six, and later he built Fort Charlotte on the Savannah River toward North Carolina.

Williamson married Eliza "Betty" Tyler of the prominent Virginia family, and they had four surviving children together. She died in 1781 during the Revolutionary War.

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. In the early 1760s, he participated in a couple of expeditions against the Cherokee, who were raiding and harassing settlers. He supported defeating and enslaving the Cherokee. 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.

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. (The British added to the star fort in the settlement, as they considered this an important strategic location and wanted to control it.) There were numerous Loyalists in this part of the state.

At this time, the settlement of Ninety Six had about 10 houses, a jail and a courthouse, with a total of about 100 settlers living in the area. In November 1775, Williamson led militia in what was called the Battle of Williamson's Fort, the first battle of the Southern Campaign here, to recapture gunpowder and ammunition hijacked by Loyalists. His call attracted 532 Patriots, but soon 1500 to 2000 Loyalists were gathering outside the village against them. The forces reached a truce and casualties were light. Williamson's men were aided by their temporary fortifications at his plantation. The open warfare tended to harden loyalties, and enmities in the interior areas became vicious.