In 1998, he moved to AJ Auxerre’s youth team. Eventually, he was promoted to the first team and he made 87 Ligue 1 appearances for them. He was part of the squad that won the Coupe de France in 2005. He also played in the UEFA Cup in each of his three seasons with the club’s first team, making a total of 17 appearances in the competition. He was also voted into the Ligue 1 team of the season for his outstanding displays at the right back position. He played in the same Auxerre team as Abou Diaby, a future Arsenal teammate, and Watford centre-back Younès Kaboul. Sagna established himself as one of the best full backs of the French top tier and thus became dubbed “Mr. Reliable” by the club’s fans. He also won Auxerre’s Player of the Year award for the 2006–07 season.

On 12 July 2007, Sagna completed a move to Arsenal from Auxerre for an undisclosed fee, thought to have been an initial €9 million that could have risen to €11 million. He was given the number 3 shirt, previously worn by former defender Ashley Cole.

Sagna made his debut for Arsenal on 19 July 2007 as a starter in the squad that beat Turkish club Gençlerbirliği 3–0 in a friendly at Austria’s Bad Waltersdorf Stadion.

Sagna received his first senior international cap for France on 22 August 2007 in a friendly match against Slovakia on the road, which they won 1–0, coming on as a substitute for François Clerc after one hour of play. He went on to make his competitive debut for France in their 6–0 away win at the Faroe Islands on 13 October 2007 in the UEFA Euro 2008 qualifiers, playing all 90 minutes. It was his only appearance in the Euro 2008 qualifying. He was not a candidate to join the French team at the Euro 2008 finals in Austria and Switzerland as he was still recovering from the injury sustained in Arsenal’s Premier League match at Chelsea in March 2008.

On 13 February 2008, exactly one week before Arsenal were set to play the second leg of their round of 16 encounter against Milan in the Champions League, Sagna’s older brother, Omar Sagna, died at the age of 28, one day before his birthday. He, however, did play the match after he was advised to by his father. Arsenal went on to win the second leg 0–2 with the same scoreline on aggregate.

After recovering from the injury, Sagna returned to the French team for their friendly match against Sweden in August 2008, playing all 90 minutes in a 3–2 away win. He went on to become a regular in the French team’s qualifying campaign for the 2010 FIFA World Cup. He was also a part of the disastrous French World Cup campaign when the team crashed out at the group stage after losing 2–1 against hosts South Africa.

Sagna scored his first goal at the Emirates Stadium for Arsenal against Celtic, a game in which Arsenal went on to win 3–2 and win the Emirates Cup for the second consecutive year. He made his 100th league start for Arsenal on 3 October 2010, a 2–0 defeat against Chelsea at Stamford Bridge. On 14 November, he scored only his second ever official goal for Arsenal in a 2–1 away victory against Everton at Goodison Park, with a powerful shot from the near post. On 8 December 2010, Sagna picked up his first red card in his Arsenal career in a 3–1 home victory against Serbian club Partizan in Arsenal’s final Champions League group game of the season.

Bacary Sagna was born in Sens, France to Senegalese immigrant parents. He is a Muslim. In 2010, he married French-Algerian model and journalist Ludivine Kadri with whom he already had a son named Elias (born in 2009). In 2013, Kadri gave birth to the couple’s second son Kais.

On 1 January 2011, in a match with Birmingham City, he was involved in an incident where Lee Bowyer stamped on him after Sagna had gone in for a tackle. Bowyer was banned by the FA for three games for the stamp. On 5 January, Sagna picked up his second red card of the season after he and Manchester City fullback Pablo Zabaleta headbutted each other in the final moments of Arsenal’s 0–0 draw with City at Emirates Stadium. On 19 January, Sagna scored his second goal of the season with Arsenal’s second goal in their 3–1 victory over Leeds United at Elland Road in a third round replay. This was Sagna’s career best for goal-scoring, his previous best coming in the 2007–08 season where scored only once. Sagna was included in the PFA Team of the Year for his performance in the 2010–11 season.

On 13 August 2011, Sagna started and played the full 90 minutes against Newcastle United at St James’ Park in a 0–0 draw on the opening day of the 2011–12 Premier League season. On Saturday 20 August, he started at left-back and played the full 90 minutes in a 2–0 defeat at the hands of a Liverpool side that picked up its first win over the Gunners away from home in 11 years. On 28 August, Sagna missed Arsenal’s game against Manchester United at Old Trafford due to illness.

On 31 March 2011, Sagna became an ambassador for Grassroot Soccer, an international non-profit that uses the power of football to educate, inspire, and mobilise communities to stop the spread of HIV.

Sagna sustained a leg break during a match against North London rivals Tottenham Hotspur after a challenge by Benoît Assou-Ekotto forced him to land awkwardly. On 29 January 2012, Sagna came off the bench against Aston Villa as an 89th-minute substitute to mark his comeback. On 26 February 2012, he started in the North London derby against Spurs, scoring a header from a Mikel Arteta cross and inspiring Arsenal to come back from being 2–0 down to go on and win 5–2. He unfortunately sustained another leg break of the same leg in the final home match of the season against Norwich City, which ruled him out of UEFA Euro 2012 in June 2012.

In October 2012, Sagna marked his return to the first team by starting in Arsenal’s 1–0 victory over Queens Park Rangers. On 3 November, he started and played the full 90 minutes at right-back against Manchester United at Old Trafford, a 2–1 league defeat. In his absence with a broken leg, Carl Jenkinson started for Arsenal and earned rave reviews, but Sagna continued to be Arsenal’s first-choice right back upon his return. On 9 February 2013, due to an injury to centre-back Laurent Koscielny during warm-ups, Sagna was forced to fill in as a central defender against Sunderland. He helped a ten-man Arsenal win 1–0. On 28 April 2013, Sagna committed a mistimed tackle inside the penalty box on Robin van Persie in a 1–1 home draw against Manchester United, conceding a penalty to bring the score level after a second-minute Arsenal goal from Theo Walcott.

Sagna missed out on UEFA Euro 2012 with a broken leg.

Sagna remained Arsenal’s first choice right-back at the start of the season, fighting off competition from Carl Jenkinson. He scored his first goal of the season, his fifth in Arsenal colours, against Stoke City on 22 September, heading in Mesut Özil’s free-kick, securing a 3–1 win for the Gunners. On 17 May, Sagna started in the 2014 FA Cup Final as Arsenal beat Hull City 3–2 at Wembley Stadium, his first trophy at the club.

It was announced on 13 June 2014 that Sagna had agreed to a deal with Manchester City to join the reigning Premier League champions after his contract with Arsenal was set to expire on 30 June. He was City’s first signing of the summer and signed a three-year deal, taking the number 3 shirt. He left the club at the end of his contract in June 2017.

On 3 February 2018, Sagna signed with Serie A side Benevento Calcio for six months and a year in option.

On 8 August 2018, Sagna signed with Montreal Impact in Major League Soccer for the remainder of the 2018 season, with an option for 2019. Although the option was not taken up, Sagna subsequently signed a one-year contract to stay with the club for the 2019 season.