Mjallby started his professional career at AIK, before moving to Celtic in 1998. During his six year career at the Scottish club, he won 3 Scottish Premier Leagues, 2 Scottish League Cups, 2 Scottish Cups, and was a runner-up in the UEFA Cup in 2002-03. He made his debut for Sweden in 1997, and went on to captain his country, He was part of the national team squad at Euro 2000, World Cup 2002 and Euro 2004.