Mjällby spent the next fourteen years in AIK and established himself in the side with solid performances as a no-nonsense tackling midfield player.

Although he was a squad member in 1992 when AIK won the league, he refused to accept the gold medal, stating that he felt he had not contributed enough.

The 1999–2000 season saw Venglos move into the role of a scout with John Barnes coming in as head coach and Kenny Dalglish as director of football.

Mjällby found himself a mainstay on the left hand side of O'Neill's three-man defence alongside Joos Valgaeren, Tom Boyd and/or Bobo Baldé.

[7] At UEFA Euro 2000, Mjällby scored for Sweden in the opening match against Belgium, taking the ball after an error by Belgian goalkeeper Filip De Wilde.

Although reducing Belgium's 2–0 lead from Bart Goor and Emile Mpenza to 2–1, Sweden failed to score an equalising goal and lost the game.

Mjällby captained Sweden at the 2002 FIFA World Cup after fellow defender Patrik Andersson had to withdraw because of injury.

Mjällby formed a solid partnership with Andreas Jakobsson when Sweden won Group F ahead of England, Argentina, and Nigeria, before being eliminated by Senegal in the second round.

[7] On 25 March 2010, it was announced that Mjällby would assume the role of assistant manager at Celtic Football Club, alongside temporary boss, and former team-mate, Neil Lennon.

[2] The appointment of Lennon and Mjällby was made after the sacking of Tony Mowbray as Celtic manager after a poor 2009–10 season for the club, culminating in a 4–0 defeat against St Mirren.

[12] On 12 October 2014, he was appointed as the Assistant Manager at Bolton Wanderers linking up again with his former Celtic colleagues Neil Lennon and Garry Parker.