[10][11] Growing up in the northern suburbs of Brisbane, Thorn played junior rugby league for Norths Aspley and Wests Arana Panthers.

I want be a good father and husband and in a footy context give my best.”[15] A few years later, in 2011, Thorn said to Stuff: “I had money, a car, a house – I thought these things would make me happy.

I felt empty ... it was only when I put my faith in Jesus that life changed for me.”[17] Thorn made his first grade debut in the NSWRL for the Brisbane Broncos, who were then defending premiers, in the 1994 Winfield Cup season's 12th round against the Canterbury-Bankstown Bulldogs.

The following year, when all players were again allowed to be selected for representative football, Thorn's debut for the Queensland Maroons came in Game I of the 1996 State of Origin series.

Thorn won his first premiership with the Broncos when they defeated the Cronulla-Sutherland Sharks in the 1997 Super League grand final in Brisbane.

[18] Following the sport's re-unification under the National Rugby League, Thorn was selected in Game II of the 1998 State of Origin series.

Thorn also played at second-row forward in the Broncos' victory at the 1998 NRL Grand Final, winning his second premiership ring.

He then played at second-row forward for the Broncos in their 2000 NRL Grand Final win over the Sydney Roosters, claiming a third premiership.

Thorn claimed another premiership ring when he played at second-row in the Broncos' 2006 NRL Grand Final victory over the Melbourne Storm.

As 2006 NRL Premiers, the Broncos travelled to England to face 2006 Super League champions, St Helens in the 2007 World Club Challenge.

During the 2007 NRL season at the Broncos' 20-year anniversary celebration, the club announced a list of the 20 best players to play for them to date which included Thorn.

[19] At the close of the 2007 NRL season Thorn switched codes once again, moving back to New Zealand to continue his rugby union career.

In 2001 Thorn moved to New Zealand and switched to rugby union, playing for the Crusaders in the Super 12 and Canterbury in the National Provincial Championship.

He thus became a dual rugby-code international, the second man in history (after Bill Hardcastle) to have represented Australia in league and New Zealand in union.

This feat was first achieved by Peter Ryan for the Brisbane Broncos in 1998 and the Brumbies in 2001, and since Thorn, only by former Crusaders teammate Sonny Bill Williams and Queensland Reds/Melbourne Storm player Will Chambers in 2012.

In 2014 a bicep injury which required surgery ended his season and called into question whether Thorn would be able to reach his stated goal of playing rugby union professionally until age 40.

In May 2015 Thorn was revealed to be a newly appointed “Development/Forwards coach” of the Australian Super Rugby team, the Queensland Reds, starting in late 2015.

Queensland Country dominated the fixture overwhelmingly (winning 57–21),[35] and, it stands as the second-largest finals victory by margin in the NRC's history.

[45] This initial successive start to the season under Thorn was a stark difference from previous years under the likes of Richard Graham, Matt O'Connor and Nick Stiles.

Unfortunately that brief success to the start of the season was overturned as the Reds lost four-straight matches,[46][47] sitting thirteenth overall after ten rounds.

In fact, the 2018 Super Rugby season saw the New South Wales Waratahs as the only Australian team to finish in a finals position.

[50][51] In April 2023, Thorn confirmed he will finish as coach of the Queensland Reds at the conclusion of the 2023 Super Rugby Pacific season.