[12] Sinfield was a member of the England squad for the 2000 Rugby League World Cup, scoring a hat-trick of tries in a 76–4 victory over Russia.

Sinfield played for Leeds at stand-off in the 2003 Challenge Cup Final against Bradford Bulls, kicking four goals in his side's defeat.

As Super League IX champions, Leeds faced 2004 NRL season premiers, the Bulldogs in the 2005 World Club Challenge.

Sinfield played for Leeds in the 2005 Challenge Cup Final, kicking four goals in their loss against Hull F.C., but was awarded the Lance Todd Trophy as man of the match nonetheless.

[14] He played at loose forward, kicking a goal in Leeds's 2005 Super League Grand Final loss against Bradford Bulls.

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.

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.

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.

[17][18] He captained Leeds from stand-off in the 2011 Challenge Cup Final defeat by the Wigan Warriors at Wembley Stadium, kicking only one goal from four attempts in their loss.

[19][20] 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 at Old Trafford.

[21][22] 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 second in the group table, with England losing out to Australia despite Sinfield kicking two goals.

Sinfield turned in a Man-of-the-Match performance in the final, winning the Harry Sunderland Trophy with a try, a penalty and four conversions.

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.

[41] Part way through the 2021 season, Sinfield left Leeds to become defence coach at rugby union club, Leicester Tigers.

[51] In October 2021, Sinfield announced his second fundraiser, running between Leicester Tigers' home ground Welford Road Stadium, and Leeds Rhinos' Headingley, a distance of approximately 101 miles (163 km), in under 24 hours.

[52] With an initial goal of £100k, Sinfield was again raising funds for the MND Association, and described it as "the toughest challenge I have ever attempted",[53] and the run was to be split into 24 sections of 7km.

[55] He said that he would not stop fundraising until a cure had been found for the disease,[56] and described himself as "massively blown away" by the amount raised,[57] but said ideas of a knighthood were "really nice but we're a team".

[61] In addition to Burrow, Sinfield said he was now also fundraising in honour of former rugby union player Doddie Weir, and former footballer Stephen Darby, both diagnosed with MND.

[69] Sinfield became the first rugby league player to be nominated for the BBC Sports Personality of the Year Award, finishing in second place behind Andy Murray in 2015.