Sinckler was named in the England under-20 "Elite Player Squad" ahead of the 2011–12 season. In his first season, he played ten times, across both the Six Nations Championship and 2012 IRB Junior World Championships. At the Junior World Championship in South Africa, Sinckler scored in pool stage victories against Italy and Ireland. He was reinstated into the squad for the 2012–13 season, playing in just three matches in the 2013 Junior Six Nations, due to his commitments with Harlequins.