In 2005 Sculthorpe scored his 1,000th point for Saints.He was named as Great Britain captain in 2005, although injury ruled out for the most of the 2005 season and prevented him from leading the side in the 2005 Tri-Nations tournament.
As one of the stars of the world game, Sculthorpe secured a lucrative sponsorship deal as the face of Gillette, which at the time was the most lucrative sponsorship deal made by any rugby league player worldwide. In March 2006, Sculthorpe signed a deal with Random House to publish his autobiography, Man of Steel, which was published in August 2007. Sculthorpe captained St Helens at second-row forward in their 2006 Challenge Cup Final victory against Huddersfield. However his time as Great Britain captain was less successful, as he only ever captained the side for 18 minutes. This came in the one off test game at Knowsley Road in 2006 between Great Britain and New Zealand, in which he was forced off after injuring his knee. Sculthorpe returned from the injury and was named man-of-the-match in St. Helens' win over Brisbane Broncos in the 2007 World Club Challenge. In September 2007 he signed a new 12-month contract with St Helens which saw him celebrate his testimonial season at the club.
Sculthorpe's final game for St. Helens was in the 2008 Challenge Cup final, where he injured his shoulder in the opening minutes. After being injury hit in 2008's Super League XIII with only a handful of games played, St. Helens announced they would release Sculthorpe at the end of the season. He was touted as a potential signing for Salford, or joining up with brother Danny at Wakefield Trinity in time for the 2009 Super League season. However, Sculthorpe announced his retirement from the game on 19 September 2008.
He was awarded the honour of MBE in the 2013 New Year's Honours List, for "services to Rugby League and to charity".