Ellis was a junior at Castleford Lock Lane prior to signing for Wakefield Trinity Wildcats' Academy side in 1999.

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

[7][8] Ellis played for Leeds Rhinos in the 2005 Challenge Cup Final at loose forward in their loss against Hull FC.

He played for Leeds Rhinos as a second-row in their 2005 Super League Grand Final loss against the Bradford Bulls.

In February 2008, Ellis agreed to a contract with Australian National Rugby League club, Wests Tigers, for a three-year deal commencing in 2009.

Great Britain forward Gareth Ellis cited a lifestyle change and the desire to test himself on the biggest stage for his reasons to quit Super League champions Leeds at the end of the new season.

[15][16][17] Wests Tigers coach Tim Sheens praised new signing Gareth Ellis on making a decision based purely on a challenge rather than money.

[19] Gareth Ellis became the first Great Britain forward to switch to the NRL since former Sydney Roosters hitman Adrian Morley spent six successful seasons in Australia.

[30] He has been capped 17 times for Great Britain and in 2007 was named in the International Team of the Year having started in 11 consecutive Tests for the Lions.

[32] In November 2008, Ellis was named at second-row in the inaugural Rugby League International Federation's (RLIF) Team of the Year, tying for the position with Anthony Laffranchi.