Born in Tulle, Koscielny began his football career playing for a host of youth clubs before moving to Guingamp in 2003, where he quickly progressed through the youth ranks, making his professional debut the following season. After becoming frustrated at being played out of position as a right back, he joined Tours for an undisclosed fee, quickly establishing himself as the team's star player in his desired position, ultimately aiding in the club's promotion into Ligue 1, while also being named in the Ligue 2 Team of the Season. Koscielny continued his impressive performances following a €1.7 million transfer to fellow French outfit Lorient in 2009, receiving multiple plaudits for his performances, largely cultivating a reputation as a clever, agile, ball-playing defender. His achievements and footballing potential attracted Arsenal, ultimately moving to the Premier League side for a fee of £8.45 million the following season.