Sumter raised a local militia group in Stateburg. In February 1776, Sumter was elected lieutenant colonel of the Second Regiment of the South Carolina Line of which he was later appointed colonel. in 1780 he was appointed brigadier general, a post he held until the end of the war. He participated in several battles in the early months of the war, including the campaign to prevent an invasion of Georgia. Perhaps his greatest military achievement was his partisan campaigning, which contributed to Lord Cornwallis' decision to abandon the Carolinas for Virginia.

During fighting in August 1780, he defeated a combined force of Loyalists and British Army regulars at Hanging Rock, and intercepted and defeated an enemy convoy. Later, however, his regiment was almost annihilated by forces led by Banastre Tarleton. He recruited a new force, defeated Major James Wemyss in November, and repulsed an attack by Tarleton, in which he was wounded.

In 1781, in response to a low number of recruits, Sumter publicly implemented a bounty for Continental Army recruiters, which stipulated that anyone who managed to recruit a certain number of volunteers for the South Carolina Line would receive Loyalist-owned slaves as a reward. Sumter acquired the nickname "Carolina Gamecock" during the American Revolution, for his fierce fighting tactics. After the Battle of Blackstock's Farm, British Lieutenant Colonel Banastre Tarletoncommented that Sumter "fought like a gamecock", and Cornwallis described the Gamecock as his "greatest plague".