Hannes played club football in Iceland for Leiknir, Afturelding, Stjarnan, Fram and KR. Hannes started his career at Leiknir at a time when the team was in the Icelandic fourth- and third tier of the Icelandic football league system. Hannes was let go from Leiknir in 2004 after making a number of serious mistakes in an important game that revolved around progressing to the second tier. During his time at Leiknir, Hannes sought to join Númi, which played in the Icelandic bottom tier, after struggling to get into the Leiknir first team, but the team rejected him. After leaving Leiknir, Hannes joined Afturelding, which played in the third tier. Early in his career, Hannes struggled with shoulder injuries. Hannes was known for poor kicking during his early career. In 2006, Hannes joined Stjarnan, which played in the second tier. He joined Fram, a first tier team, in 2007. Hannes said that at the time, he set a goal of becoming a goalkeeper for a team in the first tier in three years' time, and hoped to play abroad after that. He joined KR in 2011, winning the double with the team in his first season.
Europe
On 28 March 2012, he joined Norwegian club SK Brann on a short-term loan, as a cover for injured goalkeeper Piotr Leciejewski. In December 2013, Hannes joined Norwegian club Sandnes Ulf, having trained with the club in October 2013 as preparation for Iceland's World Cup playoff game against Croatia. On 6 July 2015, Dutch Eredivisie club NEC announced they had signed Hannes on a two-year contract. On 15 July 2016, Danish club Randers FC signed Hannes on a three-year contract. On 3 July 2018, Halldórsson signed a two-year contract with Qarabağ FK. On 5 April 2019, Halldórsson left Qarabağ by mutual consent. Hannes had at that point become the third-choice goalkeeper at Qarabag. On 9 April 2019, Icelandic club Valur signed Hannes on a four-year contract. Hannes cited family reasons and the four-year contract as reasons to return to the Icelandic top flight division.
Hannes has found some work as a film director when not playing football. He directed amongst other things Iceland's video for their entry to the 2012 Eurovision Song Contest. His employer before leaving to Norway to become a fully professional footballer, Sagafilm, has promised him his job back when he returns to Iceland after his expatriate footballing career. Hannes directed a 2018 World Cup advertisement for Coca-Cola.
