Sinfield made his first team debut for Leeds aged 16 against the Sheffield Eagles. He went on to make three further appearances during the 1997 and 1998 seasons, scoring his first try against Huddersfield in September 1998. 1999 was a breakthrough season for Sinfield, making 21 appearances and scoring 2 tries. He was unlucky not to be selected for the 1999 Challenge Cup Final at the old Wembley Stadium. Leeds defeated the then London Broncos 52–16.
Sinfield celebrated his testimonial season in 2008. He was the man-of-the-match, scoring seven points as Leeds defeated Melbourne 11–4 at Elland Road. During the season Sinfield set a club record by scoring in 63 consecutive matches. He also set a Super League record by scoring in 52 consecutive Super League games. Sinfield scored his 2,000th point for the club as Leeds defeated St Helens for the second successive year in the 2008 Super League Grand Final. Leeds won 24–16, with Sinfield successfully kicking four goals. It was the first time Leeds had ever won back to back titles. It was also the first time that a Super League team had won both the World Club Challenge and the Grand Final in the same calendar year. He was also selected for the England squad to compete in the 2008 Rugby League World Cup tournament in Australia. Group A's first match against Papua New Guinea he played at loose forward and kicked four goals from six attempts in England's victory.
During the 2009 season, Sinfield became only the second player, after Lewis Jones, to score 1,000 goals for Leeds. Sinfield capped the 2009 season by winning the Harry Sunderland Trophy for his performance as he led Leeds to victory in a third successive Grand Final. Leeds defeated St Helens 18–10, with Sinfield scoring six of the points, including a drop goal. With victory, Sinfield became the first player in history to captain four championship-winning teams. He dedicated the win to the recently deceased former Leeds player John Holmes.
Sinfield was selected to play for England against France in the one-off test in 2010.
Sinfield captained Leeds from stand-off half in the 2011 Challenge Cup final, kicking only one goal from four attempts in their loss against Wigan. However at the end of the season he extended his record to captaining five championship-winning teams after Leeds Rhinos' victory over St Helens in the 2011 Super League Grand Final. Sinfield played a pivotal role in the England team in the 2011 Gillette Four Nations, which involved games against Australia, New Zealand and Wales. They reached the Gillette Four Nations Final courtesy of coming 2nd in the group table, with Sinfield kicking two goals England lost out to Australia.
Sinfield became the all-time top points scorer for the Leeds Rhinos early in the 2012 season.
In 2012 he extended his championship tally as Leeds again came from 5th in the regular season to triumph at Old Trafford - this time 26-18 against Warrington. Sinfield turned in a Man of the Match performance in the final winning the Harry Sunderland Trophy with a try, a penalty and 4 conversions. Remarkably, he finished the 5-match playoff campaign with a 100% goalkicking record.
It was announced in January 2013 that Sinfield had won the Golden Boot - only the fourth Englishman to do so. He beat off competition from Wigan full-back Sam Tomkins as well as Australians Cameron Smith, Ben Barba, Cooper Cronk and Nate Myles to claim the prize that is awarded annually by Rugby League World magazine to the game's top performer.
Sinfield holds the record for being the highest scoring kicker in Super League history