Barrett first caught the notice of national selectors when, at the age of 19, he was named to the New Zealand Sevens team for the last two legs of the 2010 IRB Sevens World Series in England and Scotland, and he made his Taranaki debut in the ITM Cup against Northland. He became the second-youngest player to represent New Zealand 7s after Tomasi Vuolo, who was 18 years and 304 days old when he made his debut in 2009. In his first tournament for Taranaki, Barrett scored five tries and earned man of the match honors. Barrett's younger brother Ben also plays for Taranaki and they are the only siblings to have played for the club. The brothers first met while growing up in Auckland and would play rugby together during school holidays. They both attended St Peter's College before moving on to play rugby for Auckland University. While playing for Taranaki, the brothers have worked together as coaches and have helped their team achieve its best results yet. In 2015, they helped the team win the Mitre 10 Cup title after defeating Wellington 24-17 in the final at AMI Stadium. That same year, they were selected by New Zealand Rugby Union officials to travel to Australia as part of the team's training squad. Ben is three years older than James and made his first-class debut in 2014 when he came on as a late substitute in Taranaki's 26-24 loss to Canterbury.