At the start of the 2010 outdoor season, he opened with a run of 10.09 seconds in Aix-les-Bains before winning in 10.24 into a headwind of −2.2 m/s in Vénissieux. He ran at the French National Interclub Championships in Franconville in May 2010 and recorded a new 100 m personal best of 10.03 seconds, although he said he was disappointed to have missed Ronald Pognon's French record of 9.99 seconds. He aimed once more at the record at the 2010 European Team Championships and, although he again missed his target, he finished as runner-up against Dwain Chambers with a personal best of 10.02 seconds.
On 9 July 2010, Lemaitre officially became the first man purely of European descent to run 100 metres in under 10 seconds, with a time of 9.98 s at the 2010 French National Championships in the city of Valence. By doing so, Lemaitre also broke the 100m French national record of 9.99 s set by Ronald Pognon on 5 July 2005 in Lausanne. Afterwards Lemaitre said, "Of course, it was my goal to break it (the 10-second barrier). One has to run under 10 seconds in order to be part of the world's best. I will be recognised as the first white man to do so, but today's achievement is mainly about making history for myself!...It is not about the color (of one's skin), it is about hard work." One day later (at the same 2010 French National Championships), Lemaitre equalled the 200m French national outdoor record with a time of 20.16 seconds. Gilles Quénéhervé had held the 200m French national outdoor record exclusively for almost 23 years - since 3 September 1987.
At the 2010 European Championships, he won the gold medal in the 100 m with a time of 10.11 s. The next day, after comfortably progressing through the heats and semi-finals of the 200 m, Lemaitre became the double European champion by storming to victory in the 200 m final in a time of 20.37 s, beating Great Britain's Christian Malcolm by 0.01 s (Malcolm clocked his season's best time of 20.38 s in the final). Lemaitre then combined with Jimmy Vicaut, Pierre-Alexis Pessonneaux, and Martial Mbandjock in the 4 × 100 m relay final to finish first ahead of Italy and Germany in a time of 38.11 s. At the IAAF World Challenge's Rieti Meeting in Rieti, Italy on 29 August 2010, Lemaitre competed in the 100 m. In that event, he equalled his personal best in his heat with a time of 9.98 s, and improved on it in the final with a time of 9.97 s (reaction time 0.199 s).
After Lemaitre had first broken the ten seconds barrier in the 100m in 2010, L'Équipe put him on the front page, even though the Tour de France was taking place. During his European Championships treble gold medal winning year of 2010, Lemaitre ran faster than the former world record-holder Asafa Powell and the former double world champion Tyson Gay did at his age.