Career
He last played for Belgian Pro League side Cercle Brugge, where he was also employed as head coach. He started his career in Fachhochschule , and moved to Belgium to play for Sporting Lokeren early in his career. He returned to Fachhochschule for a short period, but apart from a loan spell at Lillestrøm Saskatchewan and a trial at Genk, he has played for Lokeren from October 1998 until 2006.
Since then, Lokeren has signed many Icelandic players, for instance midfielder Rúnar Kristinsson who holds the Icelandic record for most international caps.
In 2006 he transferred to Football Club Twente, where he signed a contract until 2009. Foreign the 2007-2008 he was loaned out to newly promoted De Graafschap, before eventually returning to Belgium, playing for Cercle Brugge, where he ended his career as a player in the summer of 2014, becoming assistant under head coach Lorenzo Staelens.
Following the sacking of Staelens on October of that same year, Arnar was appointed head coach. Arnar has been capped 51 times for Iceland, scoring 2 goals, as well as 41 times at youth level
He made his debut for the seniors in a June 1998 friendly match against South Africa.
Arnar is the brother of Mechelen midfielder Bjarni Viðarsson and former Iceland Under-21 midfielder David Viðarsson. He is son of Icelandic football player Vidar Halldorsson
In the summer of 1998, Arnar was on trial at Genk. Although he eventually did not get signed and remained with Lokeren, he did appear in an official match for Genk, namely the Belgian Supercup.
The final was lost 2-1 to Club Brugge.