Guðmundur started his senior team career with Víkingur Reykjavík in 1979. With Víkingur, he won six national championships and four national Cups. In 1992, he joined Afturelding where he finished his playing career in 1995.[2]
Guðmundur is ranked with Dagur Sigurðsson his fellow countryman, who is the current national team coach of Japan's Men's team and another, Alfreð Gíslason, coach of THW Kiel amongst the best coaches of our era.[dubious – discuss] He has cited former coach Bogdan Kowalczyk as a major influence.[3]
In October 2013, it was announced that Guðmundur would take over as the head coach the Danish men's national handball team on 1 July 2014, replacing Ulrik Wilbek .[4] On 21 August 2016 Guðmundur led his team to the gold medal at the 2016 Summer Olympics in Rio de Janeiro, Brazil.[5] During the Olympics, Wilbek made two attempts to gain the consent of the Danish players to fire Guðmundur, once during the tournament and then the day after Denmark won the gold.[6][7] He announced in November 2016 that he would not renew his contract, which was set to expire on 1 July 2017.[8] After his departure from the Denmark team, Guðmundur criticized his predecessor Wilbek's, who was then the head of the Danish Handball Federation, repeated attempts to undercut his management of the team during the Olympics.[9]
In 2018 he returned to coach the Icelandic national team for the third time. He signed a contract for three years.[11] On 11 January 2020, he faced Denmark for the first time since his departure and guided Iceland to a 30–31 victory.[12]
In February 2020 Guðmundur took over as a coach at MT Melsungen in the German Bundesliga. He signed a contract to the end of the season but remained coach of the Icelandic national team.[13] In September 2021, he was sacked after 3 league matches only[14]
On 21 February 2023, Guðmundur resigned as the manager of the Icelandic national team after five years at the helm.[15]
