A Great Britain and England international representative centre, he is considered one of the greatest players of the Super League era.

[9][10][11] During his career he won every honour in the British domestic game and held the records for the most Super League appearances (413) and tries (199), until overtaken by Paul Wellens, and his former Leeds Rhinos team mate Danny McGuire, respectively, in 2012.

Senior played at centre in Sheffield Eagles' 17–8 victory over Wigan in the 1998 Challenge Cup Final during Super League III at Wembley Stadium, London on Saturday 2 May 1998.

He bettered that tally in 1998, when he scored 19 tries as the Eagles shocked the Rugby league world, beating the Wigan Warriors 17–8 in the Challenge Cup Final at the old Wembley Stadium.

After a quiet start to his Rhinos career, Senior burst into life in 2000 finishing the season as joint top try scorer with 18, including four in a single match against the then Halifax Blue Sox.

In 2004 Senior made 30 appearances and scored 13 tries as the Rhinos ended their 32-year wait for the Championship, beating the Bradford Bulls 16–8 in the 2004 Super League Grand Final.

[citation needed] Senior was selected in the Great Britain team to compete in the end of season 2004 Rugby League Tri-Nations tournament.

On 23 December 2004 the Rugby Football League fined Senior along with Leeds' teammate Ryan Bailey £1,500 each after they tested positive to the banned stimulant ephedrine.

[12] As Super League IX champions, the Rhinos faced 2004 NRL season premiers, the Canterbury-Bankstown Bulldogs in the 2005 World Club Challenge.

[13] In January 2007 Senior celebrated his Testimonial match for the Rhinos with a 50–0 thrashing of his home town club, the Huddersfield Giants.

[citation needed] Senior was an integral part of the Rhinos squad that won the Super League Grand Final for three consecutive seasons in 2007, 2008 and 2009, all against St. Helens.

On 7 July 2011 it was announced Senior was to join the Crusaders on a 2-year deal from 2012 making 2011's Super League XVI his last season for the Leeds Rhinos.