Born in Oxford, Oxfordshire, Smith's father raced in Motocross, and the family owned an open practice track. Smith got his first motocross bike aged six.
Smith applied for selection to the Auto-Cycle Union (ACU) academy for young racers, but was never chosen. He spent a year in 2004 aged 13 competing in Aprilia Superteens competition, finishing fourth behind three ACU Academy riders. In January 2005 he was asked to compete for the year in the Spanish 125cc championship as part of the MOTO GP Academy, winning the last three races ending the year 2nd just one point behind the champion.
For 2006, he joined the Repsol Honda 125cc Grand Prix World Championship squad, managed by Alberto Puig finishing 19th overall and rookie of the year, with a best result of 8th place in Motegi.
Remaining with the team in 2007, he took a first podium with third place at Le Mans. Nine more top-10 finishes left him 10th overall.
Smith signed with the Polaris World Aprilia team for the 2008 season. He secured his first pole position at the opening round of the season at the Losail International Circuit in Qatar on 8 March, but suffered a technical problem while leading. A number of midseason injuries affected him, but he scored podium finishes at Jerez and Le Mans (after briefly leading the five-lap sprint required by a red flag for rain). He led at both Assen and Sachsenring, but finished off the podium in both. He ended the season sixth overall, with three poles and four podium finishes but no victory.
For the 2009 season, Smith signed for Jorge Martínez's Bancaja Aspar Racing Team to continue to ride an Aprilia, alongside Julian Simon. The deal also included an option to race in 250s for the team in 2010. Smith had almost signed for the team in 2008, before signing for Polarisworld.
Smith won his first ever Grand Prix with a dominant performance at Jerez in May 2009. Smith's first Pole Position of the season came at Mugello in which he scored his second Grand Prix victory after a brilliant fight against Nicolás Terol and Julian Simon. This race put Smith in the lead of the Championship with 74.5 points, just 3.5 points ahead of Simon. However, Simon hit form and ultimately won the title. Smith finished second to his teammate in the last three races of the season, losing the lead on the final lap in two of them. He ultimately finished as series runner-up.
In August 2009, Smith stated his ambition to move up to the new Moto2 class with the Aspar team, but the team's two seats were given to Julian Simon and Mike Di Meglio instead. In November 2009, Smith agreed a deal to remain with the Aspar team in the 125cc class for 2010 alongside Nicolas Terol.
Smith was a favourite for the title in 2010, after finishing runner-up in 2009, however the year was dominated by Spanish trio Marc Márquez, Pol Espargaró and Nicolas Teról. Smith did however manage three pole positions and six podiums, including a win at the final round of the season in Valencia on the way to fourth place in championship standings.
After the 2010 Portuguese Grand Prix, it was announced that Smith would be competing in Moto2 in 2011, racing for the Tech 3 team, alongside one of the riders that had denied him a Moto2 move a year previous, Mike Di Meglio. On 19 September 2011, it was announced that Smith had signed a three-year contract with Tech 3 that would see him remain in Moto2 in 2012, before moving to MotoGP in 2013.
Smith started his debut campaign in the Moto2 class well with seventh in his first race before following it up with a fourth-place finish at the Spanish Grand Prix. Smith scored his first podium with a second-place finish in his home Grand Prix at Silverstone, and he followed it up with two third-place finishes at the next two rounds. Smith finished his début season in the class, in seventh place in the final championship standings.
Smith's 2012 season was full of consistent points-scoring finishes, scoring points in all bar two races, but without a podium. His best result was a fourth-place finish at the Italian Grand Prix, and he eventually finished the season in ninth place in the final championship standings.
Smith was promoted to the premier class aboard a Yamaha for Tech 3, with teammate Cal Crutchlow. He concluded the season in 10th place, with 116 points, running consistently within the top ten riders in most races. His best result were a trio of sixth places obtained in the Catalan, German and Australian Grands Prix.
In 2014, Smith continued to race with Tech 3, with teammate Pol Espargaró. He finished fifth in the second round of the championship, the Grand Prix of the Americas in Texas, achieving his best result at that time. Later in the season, Smith achieved his first podium in MotoGP at Phillip Island, finishing in third place; he took advantage of late-race crashes for Cal Crutchlow and Espargaró to move into the position. He also received a penalty point from Race Direction, after he was deemed to have overtaken another rider under yellow flags.
Remaining with Tech 3 for the 2015 season, Smith finished each of the first twelve races in the top ten placings, with two fifth-place finishes being his best results. At the San Marino Grand Prix, Smith achieved his best MotoGP result with a second-place finish despite staying out on track on slick tyres as rain was falling – he fell as low as 21st in the race order before riding through the field. With Scott Redding finishing third, the duo became the first pair of British riders to finish on a premier class podium since Barry Sheene and Tom Herron did so at the Venezuelan Grand Prix in 1979.
* Season still in progress.
(key) (Races in bold indicate pole position; races in italics indicate fastest lap)
* Season still in progress.