James was named after both his paternal grandfather Jim McCartney and father (whose birth name is James Paul McCartney), as well as Linda's late mother, heiress Louise Linder Eastman. He spent the first two and a half years of his life on the road with his older sisters and his parents, who were in a successful rock group, Wings. After the band broke up in 1980, the McCartneys settled down with their children, who attended local public schools. In 1989, McCartney, along with his older sisters Mary and Stella, again joined his parents on their world tour. He continued his education with a tutor while on the road.

In 1993, celebrating his sixteenth birthday, McCartney and his friends went out to swim in the sea. When the weather started to get bad, the water currents carried him off. Paul, Linda, and Stella rushed to the site, distraught and helpless. Luckily, McCartney was recovered back to shore safely.

On 17 April 1998, in Tucson, Arizona, James McCartney, along with his father and sisters, was at his mother's side when she died from breast cancer, which had been diagnosed in 1995. Later that year, McCartney graduated from Bexhill College, near his home in East Sussex, where he pursued studies in A Level Art.

James is devoted to his family and famously appeared in support of Stella at her 1999 fashion show (her first show following the death of their mother) seated in their mother's place next to his father.

In 1995, McCartney introduced his older sister Mary to television producer Alistair Donald, the man she would later marry. He has three nephews and one niece: Mary's two sons Arthur Alistair Donald (born 1999) and Elliot Donald (born 2002); Stella's son Miller Alasdhair James Willis (born 2005) and her daughter Bailey Linda Olwyn Willis (born 2006).

In 1999, McCartney, who is a vegan, wrote a letter to the Governor of Wisconsin to try to prevent the Dairy Expo company from selling cows' milk, which he believes was intended for calves and not for humans to drink. This activism is reminiscent of his late mother, and is shared by his father and sisters.