Smith made his debut for Manawatu in the 2008 Air New Zealand Cup at the age of 19, making several substitute appearances for the Turbos. This included the historic 25–24 win over Canterbury in Round 1 and the 38–38 draw with Waikato in Round 3. For the 2009 Air New Zealand Cup, Smith established himself as the first-choice half-back for Manawatu, starting all 13 games over the course of the season and scoring his first representative try. He continued as the starter in the 2010 ITM Cup, and established himself as one of the top half-backs in the competition and earning a Super Rugby contract. In the 2011 ITM Cup, Smith continued to improve and had a successful season, scoring five tries helping the improved Manawatu squad which reached the Championship final.
Super Rugby
Smith was included in the Blues wider training group for the 2010 Super 14 season, but did not see any game action for the Auckland-based side. For the 2011 Super Rugby season, Smith signed for the Highlanders, coached by Jamie Joseph, who had previously coached Smith with the New Zealand Māori. Smith had a successful debut season, beating out Sean Romans for the job as backup to starting half-back Jimmy Cowan, and made 12 appearances including 3 starts.
International career
On the back of his strong performances for Manawatu, Smith, of Ngāti Kahungunu descent, was selected to the New Zealand Māori for their 2010 Centenary Series. He came on as a substitute against the New Zealand Barbarians after Chris Smylie suffered a fractured cheekbone, and started in the squad's historic victories over Ireland and England. On the back of a successful Highlanders season Smith made his All Blacks debut on 9 June 2012,. By the time of the 2013 season Smith had become the first choice halfback for the national team. Smith has been selected for every New Zealand squad since 2013, including the 2015Rugby World Cup squad where he started in all the knockout games. Smith became the leader of the All Blacks' haka in 2016, following the retirement of Keven Mealamu and Liam Messam's decision to swap to New Zealand'srugby 7s squad, first leading the Haka against Wales at Eden Park. Smith played his 50th international test match in the final test against Wales, during the 2016 tour, alongside lock Brodie Retallick, with whom he debuted. Although Smith lost his place as the All Blacks' Haka leader later in the season due to off-field controversy, as well as strong competition for a starting place, from TJ Perenara, Smith was retained as a regular starter into the 2017 season, when he started in all three tests against the touring British and Irish Lions. Smith became the most-capped scrum-half in New Zealand's history in the 2018 season, breaking Justin Marshall's long-standing record on 17 November 2018. Despite the ongoing competition for his starting place through TJ Perenara, Smith was chosen to start against Ireland. Smith, during his record-breaking performance, unfortunately failed to perform well, with the All Blacks losing 9-16.
Honours
Rugby World Cup: 2015
The Rugby Championship: 2012, 2013, 2014, 2016, 2017, 2018