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.
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. Lafayette observed, "It was not his fault that he was not killed or wounded [at Brandywine,] he did everything that was necessary to procure one or t'other." Laurens behaved consistently at the Battle of Germantown, in which he was wounded on October 4, 1777:
Two days after the Battle of Germantown, on October 6, 1777, he was given his official appointment as one of General Washington's aides-de-camp, and was commissioned with the rank of lieutenant colonel. From November 2 to December 11, 1777, Washington and several aides, including Laurens, were quartered at the Emlen House, north of Philadelphia in Camp Hill, which served as Washington's headquarters through the Battle of White Marsh.
Laurens's only child, their daughter Frances Eleanor Laurens (1777–1860), was born c. January 1777 and baptized on February 18, 1777. Laurens' father-in-law wrote to him that the infant had "undergone much pain, & misery by a swelling in her hip, & thigh, I believe from a hurt by the carelessness of the nurse". Fanny was not expected to live, but by July 1777, she had recovered from a successful surgery to her hip. At the age of eight, after the loss of both parents, Fanny was brought to Charleston in May 1785, and was raised there by John Laurens's sister Martha Laurens Ramsay and her husband. Against the wishes of the Ramsays, Fanny eloped in 1795 with Francis Henderson, a Scottish merchant. Later in life, she was married to James Cunnington, and died in South Carolina at the age of 83.