Isaac John represented the New Zealand under-16s side in 2004 and played for the Junior Kiwis two years later in 2006. He is a product of the New Zealand Warriors development programme, signing for them as an 18 year old in 2007. In 2008, John featured for the Auckland Lions, a reserve team for New Zealand Warriors, in the New South Wales Premier League. The following year he played and scored for Junior Warriors in the inaugural Toyota Cup competition but the Warriors were beaten by a last minute Brisbane try to prevent them reaching the final of the competition. He also made appearances for the Ontahuhu Leopards in the Auckland Rugby League competition before joining the senior New Zealand Warriors squad full time in 2009.
That year he appeared for the Auckland Vulcans in the NSW Cup and Howick Hornets in the Auckland Rugby League competition. In mid 2009 it was announced that New Zealand Warriors had re-signed John until the end of the 2011 season with an option to extend. He made his debut for the senior team on July 19th against Sydney Roosters.
Over the following two seasons John found regular first team rugby difficult to come by due to injury and competition from other players to break into the first 13. He made just 9 appearances for the Warriors between 2009 and 2011, scoring one try for the club.
On 29th October 2011, it was revealed that John would be the newest addition to Richard Agar’s revamped squad for the 2012 season after Agar saw him play for the Warriors. John revealed his limited opportunities at Warriors and conversations with Chief Executive James Elston had sold the club to him.
|23/03/2015||00:00||NRL||Sydney Roosters||20 - 12||Penrith Panthers|
|28/03/2015||00:00||NRL||Newcastle Knights||26 - 14||Penrith Panthers|
|06/04/2015||00:00||NRL||Penrith Panthers||10 - 30||North QLD Cowboys|
|11/04/2015||10:30||NRL||Penrith Panthers||22 - 12||Manly Sea Eagles|
|18/04/2015||00:00||NRL||Gold Coast Titans||32 - 6||Penrith Panthers|
|26/04/2015||00:00||NRL||Penrith Panthers||26 - 18||Cronulla Sharks|
|08/05/2015||00:00||NRL||Brisbane Broncos||8 - 5||Penrith Panthers|
|02/08/2015||07:00||NRL||South Sydney Rabbitohs||20 - 16||Penrith Panthers|
|08/08/2015||10:30||NRL||Parramatta Eels||10 - 4||Penrith Panthers|
|15/08/2015||08:30||NRL||Penrith Panthers||24 - 10||New Zealand Warriors|
|20/08/2015||10:45||NRL||St George Illawarra||19 - 12||Penrith Panthers|