Like many of today's racing drivers, Rothrist-born Leimer started his career in karting back in 2003, when he won the Swiss Junior Championship.

In the Eurocup series, he scored points in three races and took a single fastest lap at Donington Park to end the year in seventeenth place.

[1] In the Italian championship, Leimer scored points in eleven of the fourteen races and took two podium places (at Spa-Francorchamps and Valencia) to be classified eleventh in the standings.

In a very successful debut season, he finished as runner-up to champion Chris van der Drift, taking three race wins at Estoril, Imola and Monza along with five other podium places.

[3] In the accompanying Formula Master Italia series, Leimer finished in fourth place with one race win (at Mugello) from two starts.

[5] During the first six rounds of the season, he took race wins in Pau, Valencia, Brno, Brands Hatch and Spa-Francorchamps, the latter event supporting the 2009 Belgian Grand Prix.

[7] As a result of finishing championship runner-up in the 2008 Formula Master season, in November of that year, Leimer took part in a prize GP2 test with Italian team Trident Racing at the Paul Ricard circuit in France,[8] and a year later he tested with the DAMS team at the same venue as a prize for winning the Formula Master title.

[11] At the first round of the season in Barcelona, Leimer finished eighth in the feature race, a result which gave him pole position for the reverse grid sprint event which he won from the Rapax Team entry of Luiz Razia.