Squillaci began his football career playing for hometown club FC Seynois in the commune of La Seyne-sur-Mer.

He also featured in European competition for the first time in his career and, in the 2003–04 season, played on the Monaco team that reached the 2004 UEFA Champions League Final.

In 2010, Squillaci signed a three-year contract with English club Arsenal where he also reached the 2011 Football League Cup final.

[10] With Toulon in a relegation battle, Squillaci made substitute appearances in wins over Lille, Le Mans, and Lorient and defeats to Martigues and Red Star Paris.

[11][12][13][14] Toulon ultimately finished the campaign in 20th place, which resulted in the club's relegation to the Championnat National, the third level of French football.

[citation needed] Due to not being under a professional contract while playing with Toulon, Squillaci was allowed to leave the club on a free transfer.

Following his signing, Squillaci was inserted onto the club's reserve team, which was playing in the Championnat de France Amateur, the fourth level of French football.

After failing to establish himself under Claude Puel, Monaco officials announced that Squillaci would be heading to Corsica to join Ligue 2 club AC Ajaccio on loan.

[citation needed] After the departure of Deschamps and a disappointing 11th-place finish after years of European qualification, Squillaci opted to play elsewhere.

The compensation Toulon was seeking was related to a solidarity contribution in which 5% of any future transfer fee was allocated to the youth clubs who trained a player from ages 12–23.

He took the number 18 shirt, previously held by fellow French former Arsenal centre-backs Mikaël Silvestre and Pascal Cygan.

Following an injury to centre-back Thomas Vermaelen, Squillaci formed a central defensive partnership with fellow new signing Laurent Koscielny.

[26] During the 2010–11 season, Squillaci made 28 appearances and scored 2 goals for the Gunners across all competitions as Arsenal also reached the League Cup final.

He made his first Premier League appearance of the season on 2 January 2012 when he replaced midfielder Aaron Ramsey for the last ten minutes against Fulham after centre-back Johan Djourou was sent off.

[29] He made his debut in the opening match of the season, a 2–0 loss against Nantes, and since established himself in the starting 11 that helped the club place in the top ten in Ligue 1.