Martin's accomplishments since turning professional in 2008 include winning a stage at the 2011 Vuelta a España, winning a stage at the 2013 Tour de France, and winning the overall classification at the 2010 Tour de Pologne and the 2013 Volta a Catalunya. In one-day races, he won the 2010 Japan Cup, the 2010 Tre Valli Varesine, the 2011 Giro di Toscana, the 2013 Liège–Bastogne–Liège, and the 2014 Giro di Lombardia.
Martin was born on 20 August 1986 in Birmingham, England, United Kingdom. Martin is the son of Neil Martin, a former British professional cyclist, and Maria Martin (née Roche) from Ireland, the sister of 1987 Triple Crown winner, Stephen Roche, father of professional cyclist Nicolas Roche. He was born five weeks premature and suffered from asthma as a child. Martin grew up in Tamworth, Staffordshire, and was educated at St Francis of Assisi Catholic Technology College. Martin became the British under-18 national road race champion in 2004. However, in 2006, Martin decided to ride for Ireland. In 2005, he joined the French amateur team Vélo-Club La Pomme Marseille, and won the mountains classification in the Ronde de l'Isard.
Martin turned professional in 2008 with Garmin–Chipotle–H30. In 2008, he won the Route du Sud and the Irish National Road Race Championships. In 2009, Martin reached a UCI World Rankings of thirty-fifth (137 points), and rode his first Grand Tour, the 2009 Vuelta a España. In 2010, he won his first UCI ProTour stage race, the Tour of Poland. He finished forty-ninth in the 2010 UCI World Rankings (106 points); Ireland placed seventeenth in the national rankings (254 points).
2011 was Martin's break-out season. He won his first Grand Tour stage during the Vuelta a España, and became the first Irishman to hold the mountains classification; he wore the KoM jersey for stages ten and eleven. Martin won Stage 6 and finished second overall, behind Slovak Peter Sagan of Liquigas–Cannondale, in the Tour of Poland. With a second place, behind Leopard Trek's Oliver Zaugg of Switzerland, in the season-ending Giro di Lombardia, Martin finished in ninth in the UCI World Rankings (286 points); Ireland placed thirteenth in the national rankings (319 points).
Whilst achieving no wins in 2012, Martin placed sixteenth in UCI World Ranking (196 points); Ireland finished sixteenth in the national rankings (259 points).
In March 2013, Martin won Stage 4 of the Volta a Catalunya; his fourth World Tour victory. Martin gained the leader's jersey the following day, and won overall classification four days later. In April, Martin finished fourth in La Flèche Wallonne; the following weekend, he won Liège–Bastogne–Liège, beating Team Katusha's Joaquim Rodríguez of Spain. Martin's form continued at the Tour de Suisse where he placed eighth overall.
Martin, along with his teammates Andrew Talansky and Ryder Hesjedal, rode the 2013 Tour de France as co-captains. On Stage 8, finishing at Ax 3 Domaines, Talansky and Martin finished together, twelfth and thirtieth, respectively, to sit twelfth and thirteenth overall. The following day, Martin won Stage 9 in Bagnères-de-Bigorre, after escaping from the lead group with Dane Jakob Fuglsang (Astana) on the final climb. He out sprinted Fuglsang in the final kilometre to win the stage. Martin moved up to eighth overall in the process. However, Martin fell ill in the final week, and following the final three mountain stages, slipped to thirty-third overall.
Martin completed the 2013 season with a fourth-place finish in the Giro di Lombardia, and a second-place finish in the Tour of Beijing. Martin finished sixth in the UCI World Rankings (432 points); Ireland placed tenth in the national rankings (568 points).
Martin finished second in La Flèche Wallonne, behind Spaniard Alejandro Valverde (Movistar Team). Martin looked well positioned in Liège–Bastogne–Liège, sitting in second place, but in the final two-hundred meters he crashed; Australian Simon Gerrans (Orica–GreenEDGE) was victorious.
Martin's primary focus was the Giro d'Italia, which started in Belfast. However, in the opening team time trial, Martin crashed, breaking his collarbone; he subsequently abandoned the race. He skipped the Tour de France to focus on the Vuelta a España; he placed seventh overall. In October, Martin won the Giro di Lombardia; he attacked in the final kilometre, and soloed to the finish, winning by one second. Martin won Stage 4, and placed second overall, behind BMC Racing Team's Philippe Gilbert of Belgium, at the season-ending Tour of Beijing. Martin finished ninth in the UCI World Rankings (316 points); Ireland placed fourteenth in the national rankings (357 points).
In March, Martin came in tenth position of the Volta a Catalunya. He participated to Liège–Bastogne–Liège, but was caught in a pile-up and had to abandon all hopes of winning the race. He participated to the Tour de Romandie (finishing 104th) where he complained of chest pain, but only after the race was it found that he had two broken ribs, a result of his crash at Liège.
On Stage 11 of the 2015 Tour de France, Martin rode across a three-minute gap on the Col d'Aspin to the breakaway. He then led over the top of the climb. However Rafal Majka, part of the breakaway, attacked the group on the Col du Tourmalet. Martin went over the climb in third place. He rode with Emanuel Buchmann for a while before going solo to catch Majka. He passed Serge Pauwels but he couldn't quite get to Majka. He won the Combativity award for the stage.
Martin joined Etixx–Quick-Step on a two-year contract from 2016, with a focus on strengthening the team's squad for the Ardennes classics and competing as a contender in stage races.
Martin enjoyed success in his first race with his new team, winning the second stage of the Volta a la Comunitat Valenciana, his first win for over a year. He took another win against a strong field in the Volta a Catalunya, going on to finish third overall. He went on to Belgium to race in the Ardennes classics, where his best result was a third place at La Flèche Wallonne.
Martin's first significant result of the season was a stage win at the 2017 Volta ao Algarve. In April, he placed second to Alejandro Valverde in both the 2017 La Flèche Wallonne and the 2017 Liège–Bastogne–Liège. In June, his late attack in the final stage lifted him to the podium, in third place, of the Critérium du Dauphiné, overtaking Chris Froome by a single second.
In the 2017 Tour de France, Martin was involved in a crash with Richie Porte in stage 9. Despite back pain, he went on to finish the race sixth in the general classification, only learning afterwards that he had fractured two vertebrae in the crash. The injury forced him to miss the 2017 Clásica de San Sebastián.
Martin holds both British and Irish citizenship. Since turning professional, Martin resides in Girona, Catalonia, Spain. He is married to British distance runner Jess Martin.