Märtin developed his WRC career with the Ford World Rally Team, initially as an understudy to established stars Colin McRae and Carlos Sainz. He came to wider attention during the 2002 season when he led the Acropolis Rally Greece before suffering two punctures, which dropped him behind the more experienced McRae. He returned to win the Acropolis Rally the following year, in 2003, establishing himself as a race winner at the top level.
His 2003 victory in Greece was significant: Märtin became only the third driver in WRC history to win Rally Finland, the so-called Home of Rallying, breaking the Nordic stranglehold on a race that Scandinavian drivers had dominated since the championship's inception. This result confirmed him as a driver of the highest calibre on all surfaces.
The 2004 season was Märtin's finest. Still with Ford, he won the inaugural Corona Rally Mexico — a new addition to the WRC calendar — and also took victories on the asphalt rallies of Corsica and Catalunya. These three wins placed him third in the drivers' championship behind Sébastien Loeb and Petter Solberg, and marked the high point of the Märtin and co-driver Michael Park partnership.
For 2005, Märtin left Ford for Peugeot and its 307 WRC. Despite the competitive disadvantage of learning a new car, the partnership with Park had accumulated four podium finishes by the time they arrived at Wales Rally Great Britain. During the final leg of the event, on Stage 15 at Margam Park, Märtin's Peugeot crashed into a tree. The Estonian escaped without serious injury, but co-driver Michael Park was killed in the impact — the first fatality of a competitor at WRC level for over a decade. Märtin immediately withdrew from the remaining four events of the season.
The death of Park profoundly affected Märtin. He announced that he would not return to full WRC competition and indicated he would decline top-level driving offers in the future. He subsequently directed his attention towards managing a junior rallying team in the Estonian rally scene.
Despite his stated intention to step away, Märtin made a limited competitive return at the 2006 Rally of Portugal, an invitational candidate event where he drove a Subaru Impreza in Group N specification. He also participated in the 2006 Danish Touring Car Championship.
Märtin signed as official test driver for the Subaru World Rally Team in 2008, and has since test-driven for Ford and Mini. In 2009 Ford hired him as an R&D factory driver, a role that included helping prepare WRC rookie Ken Block for the 2010 season. In July 2010, Märtin competed at Rally Estonia, an IRC candidate event, in a Ford Focus RS WRC 03, winning all twelve stages. He remains active in the Estonian rally scene through MM Motorsport, a team he founded in 2005 that has won several Estonian Rally Championship titles and operates a fleet of Ford Fiesta R5 cars for hire.
Märtin lives in Monaco. He appeared in the 2019 documentary Ott Tänak: The Movie and in the 2023 film Legends of the Winding Roads. He was close friends with Richard Burns and was in the car when Burns collapsed at the wheel on a UK motorway, managing to steer the vehicle safely to the hard shoulder — an act that almost certainly prevented a serious accident. Burns subsequently died from the brain tumour that caused his collapse.
Märtin's career is defined by extraordinary pace across all surfaces, three WRC wins in a single season, and the tragedy of the 2005 Wales Rally. He holds a unique place in motor racing history as Estonia's first WRC winner and one of the sport's most talented drivers of the mid-2000s era.