ca. 1709–ca. 1787
After Adair broke with Glenn in 1750, he moved to Johnston (later Dobbs, now Greene) County in North Carolina at the invitation of Governor Dobbs, his personal friend. He settled at Fairfields, a plantation home on Great Contentnea Creek named for Fairfield, Connecticut, the birthplace of his first wife, Ann McCarty, whom he had married on October 18, 1744. They had three daughters, Saranna (who married William McTyer), Elizabeth (who married John Cade), and Agnes (who married John Gibson). Adair resumed his travels and in the 1750s was trading among some tribes of the Carolinas. In the Cherokee war of 1760, Adair received a captain's commission and led the Chickasaw tribe against the Cherokee people.
Throughout his years among the different tribes, Adair kept notes. There is no record of his education, but he practiced medicine among the different tribes. He wrote that he was "well acquainted with near 2,000 miles of the American continent," mostly in what is now the southern and southeastern United States. He was Dr. James Adair but, informally, "Robert" or "Robin."
In 1763 he went back to England. Popular history claims that he became acquainted with a wealthy lady in whose London home he met Lady Caroline Keppel, daughter of the Earl of Albemarle. They fell in love, and her family, although objecting to the marriage, permitted the wedding in 1759 after a separation proved to affect Lady Caroline's health. During this separation from him she wrote the ballad, "Robin Adair." The couple had three children (one son named Robert became a member of Parliament), and George III gave Adair an appointment in the field of medicine. Although Adair returned to America, he supposedly made trips back to England. Lady Caroline died in 1769 at the age of thirty-two.
For two or three years after 1765 Adair was in America trading with the Chickasaw and Choctaw tribes out of Mobile. Late in 1768, he was in New York trying unsuccessfully to find a publisher for his book, History of American Indians, in which he tried to prove his theory that American Indian people were the lost tribes of Israel; the book was published in London in 1775. In 1770 Adair removed his family from Dobbs to Bladen (now Robeson) County, where he purchased a large plantation that he named Patcherly. He likely served as physician with Francis Marion during the American Revolution, as he lived in a Whig area where Marion often conducted military operations.
Adair was also an enslaver. His will listed the names of some of the people he enslaved. Pomp, Babby, Sam, Jack, Hannah, Nelly, and Lucy are all explicitly named by the will.
The most notable Whig of south Robeson, Archibald McKissack, witnessed Adair's will. Although by some reports Adair was buried in England in 1790, his will was probated in Bladen County in 1787. In it he left an inheritance to Robert Adair of County Antrim. The North Carolina Society of Colonial Dames was responsible for a granite shaft erected to honor him in August 1934 near Ashpole Presbyterian Church (Robeson County).