He started 2003 with a win in the Rund um Sebnitz in Germany, and he also took a podium placing on Stage 1 of the Tour de Berlin.

[9] Martens' professional career began in Qatar with the International Grand Prix Doha where he finished in second-to-last position, nearly 4' 30" down on the winner.

In October 2007, Martens announced he would ride for UCI ProTeam Rabobank from 2008 on an initial two-year contract, saying his three professional wins in two years at Skil–Shimano were impressive.

[24] He also recorded top-ten results at the Eschborn–Frankfurt City Loop (fifth),[25] the Giro del Piemonte (sixth), and a repeat of his eighth-place overall finish from the previous year, at the Sachsen Tour.

[26] He recorded his first victory with Rabobank in 2010, winning the Grand Prix de Wallonie,[27] ahead of Riccardo Riccò and Cadel Evans.

[27] In 2011, Martens recorded tenth-place finishes in two of the Ardennes classics – at the Amstel Gold Race and La Flèche Wallonne.

[29][30] At the 2012 Vuelta a Burgos, Martens took a stage victory on the penultimate day of the race, winning on an uphill finish at Clunia.

[40][41] Martens made his Tour de France début in 2015,[42] but he took no further individual wins over the next few years, with his best result coming at the 2017 Volta Limburg Classic, where he finished in fifth place, behind the lead group.