Koscielny was born in the city of Tulle and began his football career playing for a host of amateur and youth clubs such as Brive, Tulle Corrèze and Limoges. In 2003, he moved to Guingamp, where he made his professional debut the following year. Koscielny joined Tours for an undisclosed fee to play in the third tier of the French league in 2007 and enjoyed promotion with his new side before being named in the Ligue 2 Team of the Season the following campaign. In 2009, newly promoted Ligue 1 side Lorient signed Koscielny for around €1.7 million, and his performances caught the eye of Arsenal manager Arsène Wenger, who signed him in 2010. Since then, he is regarded as one of the best players in his position in the Premier League.
Koscielny was first called up to the French national team on 4 February 2011 and has since represented the country at UEFA Euro 2012, UEFA Euro 2016, and at the 2014 FIFA World Cup. Prior to his call-up for France, he was also eligible to play for Poland, as a result of his family roots.
Koscielny was born in Tulle, Corrèze. His career began in 2004 with Guingamp, a team that had produced such talents as Didier Drogba and Florent Malouda. Three years later, Koscielny was forced to take stock and drop down to the French third tier. He made 47 appearances for Guingamp, between 2004 and 2007, mostly as a right fullback.
In 2007, Koscielny joined Tours FC, deciding that he wanted a chance at centre back. He showed remarkable improvement in aerial ability and positional awareness that limited him to just three yellow cards. During the 2008–09 season, Koscielny made 34 Ligue 2 appearances, scoring 5 goals from defence, and was named in the division's best XI at the end of the season.
On 16 June 2009, newly promoted Ligue 1 side FC Lorient signed Koscielny on a four-year contract for around €1.7 million. He established himself as a starter and played in his preferred centre-back position. Over one season at the club, he scored 3 goals in 35 appearances. On 7 March 2010 Koscielny tapped in his first Ligue 1 goal against Olympique Marseille at Stade Vélodrome in the 65th minute in a 1–1 draw. Koscielny became a fan favourite when he scored his second goal with a header deep in injury time against Montpellier in a 2–2 draw after a cross from Olivier Monterrubio.
He scored another against Bordeaux after 11 minutes. However, he received a red card 15 minutes later for a foul in the penalty area against his future Arsenal teammate Marouane Chamakh. Koscielny helped Lorient to finish the campaign in 7th place; its best finish in Ligue 1. An impressive season culminated in reports from French daily Ouest-France that the defender was training with the Arsenal reserves ahead of an official announcement. On 6 July 2010, Koscielny agreed personal terms with Arsenal. Arsenal's initial offer for Koscielny was believed to be in the region of £4 million, while they were then prepared to do a deal that would also involve youngster Francis Coquelin moving on loan in the opposite direction, which Lorient rejected. During the summer transfer window of 2010 reports began to emerge in late June that a deal had been reached to bring Koscielny to Arsenal for a reported sum of £8.45 million.
Arsenal confirmed the signing on 7 July 2010 and Koscielny was handed the club's number 6 shirt, previously worn by Philippe Senderos, who had moved to Fulham earlier in the summer. Wenger explained he bought Koscielny due to the fact he added height to the team and Arsenal were low on centre backs. Koscielny made his debut against Barnet in a pre-season friendly in July 2010. One month later in his competitive debut against Liverpool at Anfield, he was sent off in the 94th minute for a second bookable offence. Both of his yellow cards were shown beyond 90 minutes. The game ended in a 1–1 draw with both teams having debut players sent off. He scored his first goal for Arsenal on 11 September 2010, in a 4–1 win against Bolton Wanderers.
Koscielny made his debut in European competition on 15 September 2010, when he partnered Sébastien Squillaci in a 6–0 win over Braga at the Emirates Stadium. Koscielny was praised for performances against Sunderland and made a brilliant tackle against Tottenham Hotspur to stop Aaron Lennon from scoring during their Football League Cup tie. On 27 October, he blocked a shot off the line and provided an assist to Theo Walcott against Newcastle United in the fourth round of the League Cup, where Arsenal went on to win the match 4–0 at St James' Park. Koscielny was given a straight red card in Arsenal's 0–1 loss at home against Newcastle United on 7 November 2010. This was his second sending off of the season.
On 25 January 2011, Koscielny scored the second goal in the 3–0 win over Ipswich Town in the League Cup semi-final second leg win at the Emirates Stadium. Throughout January 2011, Koscielny and Johan Djourou were a part of a defence that did not concede a goal in the Premier League. On 1 February, he scored his third goal of the season, which proved to be the winner in a 2–1 turn around victory over Everton. Koscielny had arguably his best performance to date for Arsenal in the first leg of the Champions League round of 16 match against Barcelona. The match ended in a 2–1 home win for Arsenal. Koscielny's performances were applauded and praised in many media reports of the game. Former Arsenal defender, Martin Keown, said his performance was "impressive" and "magnificent throughout". On 27 February, in the last minute of the 2011 Football League Cup Final, Koscielny was involved in a mix-up with his goalkeeper Wojciech Szczęsny, which resulted in Birmingham City striker Obafemi Martins scoring the winning goal.
Koscielny started the 2011–12 season alongside Thomas Vermaelen in central defence. Koscielny was injured and taken off the pitch 10 minutes into a 2–0 home loss to Liverpool on 20 August 2011. However, he returned to the starting line-up Arsenal's biggest ever Premier League defeat, losing 8–2 to Manchester United at Old Trafford. On 17 September, Koscielny scored an own goal as Arsenal lost 4–3 to Blackburn Rovers in the Premier League. He went on to be named man of the match away at Olympique Marseille in the Champions League, where Arsenal won 1–0 at the Stade Vélodrome, keeping their first clean sheet in a Champions League away game since 18 August 2009 against Celtic, where the Gunners had won 2–0.
On 29 October, Koscielny received praise for his solid performance in Arsenal's 5–3 away win against Chelsea. He scored his first goal of the season on 2 January 2012 in a 2–1 away defeat against Fulham. He scored his first Champions League goal against A.C. Milan in a 3–0 victory in the round of 16 second leg. On 13 May 2012, he scored the winning goal in a 3–2 victory against West Bromwich Albion to ensure that Arsenal finished third in the Premier League. He was voted as the team's fourth most important player by the fans in Arsenal's Player of the Season poll for his performances throughout the 2011–12 season.
On 24 July 2012, Koscielny signed a new long term contract with Arsenal. His first goal of the season was a late equaliser against champions Manchester City on 23 September 2012 in a match which ended 1–1. On 30 October, he scored a header from a corner in the 88th minute against Reading in the Football League Cup, helping Arsenal come back from 4–0 down to eventually win the game 7–5.
On 13 March 2013, Koscielny scored against Bayern Munich at the Allianz Arena in the UEFA Champions League Last 16. His goal put Arsenal 2–0 ahead on the night and level at 3–3 on aggregate, but they still exited the competition on the away goals rule.
On 19 May, Koscielny scored the winning goal as Arsenal beat Newcastle United 1–0 at St James' Park in their final Premier League fixture of the season to secure a fourth-placed finish and qualification to the 2013–14 UEFA Champions League play-off round.
Koscielny was voted as Arsenal's second best player of the season.
Koscielny started the 2013–14 season by conceding a penalty and getting sent off in Arsenal's 3–1 Premier League opening day defeat at home to Aston Villa. However, his partnership with Per Mertesacker saw the Gunners achieve the joint-best defensive record in the Premier League at the halfway point of the season, as the team topped the table.
On 22 February 2014, Koscielny scored his first goal of the season in a 4–1 defeat of Sunderland. In the team's penultimate home match, Koscielny scored his second goal of the season in a 3–0 win against Newcastle United.
On 9 May 2014, Koscielny signed a new long-term contract with Arsenal.
On 17 May, Koscielny scored Arsenal's equalising goal as the team came from 2–0 down to defeat Hull City 3–2 in the 2014 FA Cup Final, winning his first piece of silverware with the club.
On 16 August 2014, Koscielny opened his goal-scoring account for the season, scoring Arsenal's first goal in the 2014–15 Premier League. The goal was header from a free-kick in first half stoppage time against Crystal Palace at the Emirates Stadium. After sustaining an achilles tendon injury during the October international break, Koscielny was absent from the Arsenal team for seven weeks. On 29 November, he returned to Arsenal's starting line-up alongside Per Mertesacker in central defence, as the Gunners kept a clean sheet in a 1–0 victory against West Bromwich Albion at The Hawthorns. In the team's following fixture at home to Southampton, Koscielny helped Arsenal to record a second consecutive clean sheet and 1–0 win.
On 11 January 2015, Koscielny scored the first goal in a 3–0 win over Stoke City. On 18 January, he started in central defence as Arsenal shut out defending champions Manchester City to win 2–0 at the City of Manchester Stadium. On 10 February, he scored the opening goal as Arsenal defeated Leicester City 2–1. On 30 May 2015, Koscielny was selected to start in the 2015 FA Cup Final, playing the full 90 minutes in a 4–0 defeat of Aston Villa at Wembley Stadium.
Koscielny started every game in the first month of the season, excluding a 0–0 draw with rivals Liverpool at Emirates Stadium, due to an injury. Koscielny played a vital part as Arsenal won 5 games out of six in October to go second in the Premier League, with Koscielny scoring in successive Premier League wins against Everton and Swansea. With manager Arsene Wenger opting to play Gabriel Paulista ahead of vice-captain Per Mertesacker, Koscielny began appearing as captain in February, due to injuries to other captains Santi Cazorla & Mikel Arteta. This ended after a 3–2 defeat to Manchester United, when Koscielny missed four games through injury, including a North London derby at White Hart Lane. He returned in a 3–1 away defeat to Barcelona in the Champions League, as Arsenal exited the competition at the round of 16 for the sixth season running. Koscielny scored his fourth of the season in a 3–3 draw with West Ham.
Koscielny started his first game of the season in a goalless draw against Leicester City. On 10 September, he was given the role of captain and scored a fine overhead kick to equalize for the Gunners in a game which they then won 2–1 against Southampton making him Arsenal's highest goal scoring defender in EPL history, with the landmark achieved on his 31st birthday. Koscielny was involved in a controversial incident at the end of Arsenal's 1–0 win over Burnley, in which Koscielny and Alex Oxlade-Chamberlain were involved in a goal which appeared to be scored from an offside position after hitting Koscielny's arm. Neither the perceived deliberate handball or offside offenses were deemed to be infringements and the goal was later correctly credited to Koscielny. On 12 January 2017, Koscielny, alongside teammates Francis Coquelin and Olivier Giroud signed a new long-term contract with Arsenal. Two days later, he made his 200th appear for Arsenal in the Premier League on a 4–0 away victory against Swansea City.
On the last day of the 2016–17 season he received red card for a tackle on Enner Valencia of Everton. Arsenal lost an appeal and Koscielny was banned for three games, meaning he would miss FA Cup final.
Although he was absent from the FA Cup final and the Community Shield, Koscielny lifted both trophies alongside Per Mertesacker.
Koscielny was called up to the French national team on 3 February 2011. Before his call up for France, Koscielny was also eligible to play for Poland and initially considered applying for Polish citizenship. In August 2010, it was reported that he had not submitted an application and cancelled a meeting with Poland national team coach Franciszek Smuda in Warsaw on 7 August. Koscielny later declared that his priority was to play for France.
Koscielny was called up to the French team for the first time on 3 February 2011, for the team's friendly versus Brazil on 9 February. Koscielny made his first appearance for les Bleus in a friendly match against the United States on 11 November, playing the entire match in a 1–0 win.
He made his first appearance in a major tournament during UEFA Euro 2012, when he played the entire match in a 2–0 defeat to Spain in the quarter-finals but was voted Man of the Match by the French media. On 15 November 2013, he was sent off in a 2014 World Cup qualifying play-off against Ukraine.
On 13 May 2014, Koscielny was named in Didier Deschamps' squad for the 2014 FIFA World Cup. He made his FIFA World Cup debut as a substitute for the injured Mamadou Sakho in a 5–2 defeat of Switzerland in the team's second group match. He was then named in the starting line-up for the next match against Ecuador, helping the team to a clean sheet in a 0–0 draw. On 4 June 2016, Koscielny scored his first international goal in a friendly match against Scotland.
His grandfather and his great-grandfather were Polish miners who worked in the North of France. He has a brother who is 10 years older than him and played in the amateur leagues and now trains at Argentat football club. His father also played in the fourth division of French football for many clubs before becoming a manager. He is married to his long time girlfriend Claire Beaudouin.As of match played 31 August 2017
As of 3 September 2017.
FA Cup: 2013–14, 2014–15, 2016–17
FA Community Shield: 2014, 2015
UEFA European Championship: Runner-up 2016
Ligue 2 Team of the Year: 2008–09