Kristjánsson started his playing career at FH Hafnarfjörður, before moving to KR Reykjavík in 1996.
[1] Kristjánsson took charge of Icelandic premier league team Fram on 30 June 2004, replacing Ion Geolgău.
On 6 July 2006 Kristjánsson was appointed manager of Breiðablik UBK, finishing the season in 5th place.
In 2009 the club won its first big title in history with a win in the Icelandic Cup.
In April 2014 it was announced that Kristjánsson would take over as head coach of FC Nordsjælland in July 2014.