During 1775, the first year of the American Revolution, the city of Charleston in the colony of South Carolina was a rising commerce city which also was part of a springboard for recruitment of the Colonist rebels. While militia men skirmished with British and Loyalist bands through the southern colonies, the people of Charleston worked to support the rebellion by funneling recruits through the city as well as manufacturing supplies such as clothing, tools, horse saddles for the Patriot army. While the British were attempting to stem the rebellion up north, a small force was detached to secure the southern colonies which included Charleston. Maj. Gen. Henry Clinton was selected to lead the Southern Expedition, and Admiral Sir Peter Parker was named to command the expedition's Royal Naval Force gathering in Cork Harbour, Ireland.

In late January of 1776, Clinton sailed from Boston, bound for Cape Fear, North Carolina. In Charleston, John Rutledge, a member of the Continental Congress, arrived in Charleston with information of a British move into South Carolina. Named as the newly appointed president of the General Assembly that remained as the backbone of South Carolina's revolutionary government, Rutledge organized a defense force under the command of 46-year-old Colonel William Moultrie, a former militiaman and Indian fighter. Moultrie saw Sullivan's Island, at the mouth of the entrance to Charleston Harbor as a good place suited to build a fort to protect the entrance from intruding enemy warships.

Sullivan Island was chosen because at the time it was a geographic obstacle that shielded the harbor. A large vessel sailing into Charleston first had to cross Charleston Bar, a series of submerged sand banks lying about eight miles southeast of the city. Moultrie and his 2nd South Carolina Regiment arrived on the island in March, 1776 and began construction of a fortress to defend the island and channel to Charleston Harbor. The construction moved slowly in which an observer, Captain Peter Horry of the Patriot naval detachment, described the site as a "an immense pen 500 feet long, and 16 feet wide, filled with sand to stop the shot". The workers constructed gun platforms out of two-inch planks and nailed them together with wood spikes.

During late May, British frigates arrived to scout the area and observe the construction of the enemy fort on Sullivan's Island. The main British fleet arrived outside of Charleston Harbor on June 1. Moultrie observed British scout boat observing possible landing points on nearby Long Island (now called Isle of Palms) just a few hundred yards from Sullivan Island. General Charles Lee, the commander of the South Carolina patriot troops, arrived a few days later and was put in command of the land forces around Charleston. On June 8, after most of the British fleet had crossed the bar and anchored in Five Fathom Hole, Clinton sent a proclamation to the patriot rebels to lay down their arms or face military action, which Rutledge rejected the demand. With the fort on Sullivan Island only half complete, Parker was confident that his warships would blast the fort into pieces.

The square-shaped Fort Sullivan made up of only the completed seaward wall, with walls made from Palmetto logs 16 feed wide and filled with sand, which rose 10 feed above the wooden platforms for the artillery. A hastily erected palisade of thick planks helped guard the powder magazine and unfinished northern walls. A assortment of 31 hard-to-get cannon ranging from 9- and 12-pounders as well as a few English 18-pounders and French 26-pounders dotted the front and rear walls.

The British fleet to attack the fort made up of nine man-of-war ships, with the flagship being the 50-gun Bristol, as well as the ships-of-the-line Experiment, Actaeon, Active, Solebay, Syren, Sphinx, Friendship, and the bomb-vessel Thunder, all mounting nearly 300 heavy cannon. Meanwhile, Clinton had landed most of his troops on nearby Long Island to cross the shallow sandbar to attack Fort Sullivan from the rear during the upcoming attack.