Sinfield started playing Rugby League at Waterhead ARLFC. He was a player, captain and director of rugby for Leeds Rhinos where he led the team to seven Super League championships and two Challenge Cup successes. He was a loose forward as well as a stand-off.He played 26 rugby league tests for England which he captained and 14 tests for Great Britain. He holds the record for the highest points-scorer in Super League history. Sinfield served as defence coach for the Leicester Tigers alongside Steve Borthwick in their record-breaking run, before joining the England Senior Men’s setup in the same role, alongside Borthwick, in December 2022.