He started his football career at the local club ÍA Akranes, before moving to Norway at age 15 together with his family.

[4] He made his senior debut the same season, during the spring of 2008, whilst also signing a new three-year deal with the Eredivisie side.

He made his continental debut for the club in the Europa League on 18 September 2008, coming on as a late sub in a 1–1 draw against the Portuguese side Vitória.

He made his come back on 8 November in a Danish Cup fixture against Aalborg, 0–0 after full-time and 4–2 on penalties, where Arnór slotted home his kick from the spot.

[9] In addition, he had a successful run in the Svenska Cupen, scoring 2 goals in 6 fixtures as Helsingborg made it all the way to the final.

[30] Ahead of the 2015 season, with two years left on his contract, Arnór was loaned to FC Torpedo Moscow in the Russian Football Premier League.

[31] In his debut on 15 March 2015 against Zenit Saint Petersburg, he scored an added-time scissor kick to equalise for his team in a 1–1 draw.

[9] In the summer of 2015, Arnór returned to Helsingborg, only to be deemed surplus to requirements (due to his reported high salary) by then manager Henrik Larsson.

[34] Due to injuries in the squad, however, Arnór returned to the starting eleven and scored a long-range strike in a 3–1 win against AIK on 19 July 2015.

[39] A month later, Arnór scored a spectacular volley against fierce rivals AIK in the quarter final of the Svenska Cupen.

Hammarby won the game on penalties after goalkeeper Ögmundur Kristinsson, a fellow countryman of Arnór, scored the decisive kick.

[40] During his first league season at the club, Arnór quickly established himself as a regular starter, making 27 appearances and scoring 4 goals.

[9] At the beginning of 2017, under the reign of new manager Jakob Michelsen, Arnór Smárason was appointed as the new vice captain at Hammarby.