Biography from Benson J. Lossing in his Pictorial Field-Book of the Revolution [with minor edits]:

William Thomson was a native of Pennsylvania, and a relative of Charles Thomson, a secretary of the Continental Congress. He was born about the year 1727, and while a child, was taken to Orangeburgh District in South Carolina.

He was a patriot and was placed in command of the 3d regiment, called the Rangers. With his regiment, he fought in the battle on Sullivan's Island in 1776. He was with Major General Robert Howe in Georgia, and served under the command of D'Estaing at Savannah. He behaved gallantly and suffered much during the greater part of the war.

At its close, he returned to his estate at Belleville, near Fort Motte, with shattered health and fortune. There, he continued the pursuit of an indigo planter, which he began before the war, until 1796, when declining health induced him to go to the medicinal springs in Virginia. He died there on November 22, 1796, at the age of sixty-nine years.

Early in 1775, he was Colonel over the Orangeburgh District Regiment of Militia.

On June 18, 1775, he was commissioned as Lt. Colonel/Commandant of the SC 3rd Regiment (Rangers) and was soon in action at the battle of Great Cane Brake and the famous Snow Campaign. He was promoted to full Colonel on May 16, 1776 and saw action at Fort Moultrie, the Cherokee Expedition, Stono Ferry, the Siege of Savannah, and the Siege of Charleston. He was taken prisoner at the Fall of Charleston on May 12, 1780 and was on parole until the end of the war.

He was commissioned a Brevet Brigadier General on September 30, 1783. He died on Novemberr 22, 1796.