Senior began his career at the Sheffield Eagles. He was signed from Huddersfield YMCA rugby union team by Gary Hetherington in 1995. Senior played in the very first Super League match in 1996, a 30-24 defeat to Paris Saint-Germain. Senior made his Great Britain debut in 1996, scoring a try after coming off the bench in the 72-4 win over Fiji. He was also his club's top try scorer with 17 tries that season. 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. He also made his first start for Great Britain in 1998, against New Zealand.
Senior made 139 appearances for the Eagles and scored 55 tries during his five seasons at the club. Notably, two of those tries came against his future employer, the Leeds Rhinos.
Senior signed for the Rhinos on the final day of the 1999 Super League regular season. He made his debut for the club on the same day, against the London Broncos. 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.
Senior also represented Yorkshire in the now defunct 'War of the Roses' Origin Series, against cross-Pennine rivals Lancashire. Senior played in three consecutive defeats in 2001 and 2002, twice at Headingley, scoring two tries. In 2003 Yorkshire beat Lancashire. Senior scored two tries in the 56-6 victory at Odsal Stadium. In 2003 Senior was named the Leeds Rhinos Players' Player of the Year.
As Super League IX champions, the Rhinos faced 2004 NRL season premiers, the Bulldogs in the 2005 World Club Challenge. Senior played at centre in Leeds' 39-32 victory. In 2005 Senior became only the 20th player in the club's history to score 100 tries in his career. Senior played for Leeds in the 2005 Challenge Cup final at centre in their loss against Hull FC. In January 2007 Senior celebrated his Testimonial match for the Rhinos with a 50-0 thrashing of his home town club, the Huddersfield Giants. Senior scored a late try in the match. Also in 2007 Senior played in all three Test matches during the 3-0 series whitewash over New Zealand. The third and final Test of the series was the last time Great Britain would play as a team.