Laurens arrived at Charleston in April 1777. That summer he accompanied his father from Charleston to Philadelphia, where his father was to serve in the Continental Congress. Henry Laurens, finding himself unable to prevent his son from joining the Continental Army, used his influence to obtain a position of honor for his 23-year-old son. General George Washington invited Laurens to join his staff in early August, as a volunteer aide-de-camp.

Laurens became close friends with two of his fellow aides-de-camp, Alexander Hamilton and the Marquis de Lafayette. He quickly became known for his reckless courage upon first seeing combat on September 11, 1777, at the Battle of Brandywine during the Philadelphia campaign. After spending the remainder of the winter of 1777–1778 encamped at Valley Forge, Laurens marched to New Jersey with the rest of the Continental Army at the end of June 1778, to face the British at the Battle of Monmouth.[6] Near the start of battle, Laurens had his horse shot out from under him while he did reconnaissance for Baron von Steuben.

On December 23, 1778, Laurens engaged in a duel with General Charles Lee just outside Philadelphia, after Laurens took offense to Lee's slander of Washington's character. Lee was wounded in the side by Laurens's first shot and the affair was ended by the men's seconds, Alexander Hamilton and Evan Edwards, before Laurens or Lee could fire a second shot. Laurens was taken prisoner by the British in May 1780, after the fall of Charleston. As a prisoner of war, he was shipped to Philadelphia, where he was paroled with the condition that he would not leave Pennsylvania. In March 1781, Laurens and Thomas Paine arrived in France to assist Benjamin Franklin, who had been serving as the American minister in Paris since 1777. Together, they met with King Louis XVI, among others. Laurens gained French assurances that French ships would support American operations that year; the promised naval support was later to prove invaluable at the Siege of Yorktown.