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. By this time Senior held the record of playing the most Super League rounds of any player, with 223 out of a possible 237. Senior was selected in the Great Britain team to compete in the end of season 2004 Rugby League Tri-Nations tournament. In the final against Australia he played at centre in the Lions' 44–4 loss. 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.