Thomson was born in Ashburton. He attended Christchurch Boys' High School, and played in the same 1st XV side as fellow All Black Dan Carter. Carter revealed on Instagram, that Thomson was the captain during his time in the school's 1st XV. As he progressed through national rugby sides he played for:
Thomson made his debut for Otago on 17 September 2004 when he started against Waikato. Thomson played four matches in 2004, starting in all of them. In 2005 he played in five matches, starting only once. 2006 saw Thomson move to number 8 where he played five matches, including try-scoring appearances in the 56–21 win over North Harbour, and against Southland. In 2007 Thomson moved back to flanker and started in all his nine matches, scoring three tries. By the time he left New Zealand for the 2013 season, Thomson had played 50 games for Otago, scoring 13 tries. Thomson returned to Otago on 17 August 2019, coming off the bench in the Ranfurly Shield defence against Southland. Otago won the match 41-22.
2006, 2008-12: Highlanders
Thomson played in three Super 14 matches for the Highlanders in 2006, all as a replacement. He was not selected for 2007, despite having a good season with Otago in the Air New Zealand Cup. In 2008 Thomson was recalled. He began as a substitute, but played so well that midway through the season he'd become the team's top choice flanker. He played in twelve matches, scored five tries, and ultimately made the All Blacks squad that year. Thomson remained at the Highlanders until he left New Zealand at the end of 2012. He finished as the Highlanders' second-highest try-scorer with 21, behind only Jeff Wilson. Thomson scored 6 tries in each of his last two seasons with the team, including a three-try match against the Rebels in 2012.
2008-12: All Blacks
Thomson was selected for the All Blacks in June 2008, making the squad ahead of Kieran Read, who was also waiting to debut in test rugby. The same squad contested the Iveco test series and the 2008 Tri-Nations. Thomson debuted off the bench against Ireland when he replaced Rodney So'oialo. He then started in the second test against England. In 2009 he started the international season at openside flanker, filling in for captain Richie McCaw, before breaking his thumb 30 minutes into the game. Thomson was side-lined for the better half of the season before making his comeback against Australia in the final Tri-Nations match, playing at blindside flanker. He was the fastest player over 40m in the All Blacks squad of 2009. In 2010 he made the squad for the early-season Steinlager Series against Ireland and Wales, but not the Tri-Nations or end-of-year grand slam tour. Thomson was a member of the victorious 2011 Rugby World Cup squad. In total he played 29 tests and scored 6 tries for New Zealand.
2013-14: Canon Eagles (Japan)
Thomson played for Tokyo's Canon Eagles for two years. On 1 November 2014, Thomson played for the Barbarians, scoring a try against Australia at Twickenham stadium in London.
2015: Return to Super Rugby - Reds and Rebels (Australia)
In 2015 Thomson returned to Super Rugby with the Queensland Reds. He played 15 matches in the season, finishing as the team's top tackler and the competition's second-most prolific winner of opposition lineouts. On 20 July 2015, Thomson announced that had signed with the Melbourne Rebels for 2016.
2016 return to Canon Eagles and illness
In 2016 Thomson resigned with Japan's Canon Eagles for the 2016-17 season. In late 2017, Thomson was hospitalised with a mystery illness, later found to be lumbar discitis. Thomson was in hospital for two months before being released.
Nickname
He is known by supporters as the "wooly mammoth" due to his size and hair.
Arrest
Thomson was arrested for an alleged assault at a Havelock North motel in December 2008. Charges were dropped at a depositions hearing in July 2009.
