A midfielder, Kolbeinn began his career in his native Iceland in the Fylkir academy. His progression was such that he became the club's youngest-ever player when he made his debut at age 14 years and 229 days in a 3–0 group stageDeildabikar victory over Þróttur on 11 April 2014. Kolbeinn signed a new -year contract on 2 May 2015, he became the youngest player in the club's history to start a Úrvalsdeild match, when he made a late substitute appearance in a 1–1 draw with Fjölnir. Kolbeinn made 15 appearances during the 2015 season and departed the Fylkisvöllur in January 2016.
On 27 October 2015, Kolbeinn moved to the Netherlands to join Eredivisie club FC Groningen on a -year contract, effective 1 January 2016. The transfer made him the youngest player to sign a contract with the club. He began his time with the club in the U17 and U19 teams, before progressing into the U23 team, for whom he made 24 Derde Divisie appearances during the 2016–17 and 2017–18 seasons. Kolbeinn was released when his contract expired at the end of the 2017–18 season.
On 11 June 2018 it was announced that Kolbeinn had moved to England to join the B team at Championship club Brentford. He signed a two-year contract, with the option for a further year, effective 1 July 2018. After making his international debut for Iceland in January 2019, Kolbeinn became Brentford B's first ever player to win an international cap without having yet made a first-team appearance for the club. On 17 February 2019, he won his maiden first team call up for an FA Cup fifth round match versus Swansea City and remained an unused substitute during the 4–1 defeat. Kolbeinn finished the B team season with 43 appearances and 9 goals and departed on loan in May 2019. After returning to Griffin Park and playing with the B team in Denmark in early August, he departed the club.
Return to Fylkir (loan)
On 3 May 2019, Kolbeinn returned to Úrvalsdeild club Fylkir on a two-month loan, which was subsequently extended by a further month. He made 15 appearances and scored two goals during his spell.
Borussia Dortmund
On 20 August 2019, Kolbeinn moved to Germany to transfer to the reserve team at Borussia Dortmund on a three-year contract for an undisclosed fee. He finished a shortened 2019–20 season with 17 appearances and as of May 2020, he was training with former club Fylkir.
International career
Kolbeinn has been capped by Iceland at U16, U17 and U19 level and is a current U21 and senior international. He was a part of the U16 squad which won the bronze medal at the 2014 Summer Youth Olympics. Kolbeinn received his maiden call into the senior squad in January 2019 and made his debut as a substitute for Guðmundur Þórarinsson after 78 minutes of a 2–2 friendly draw versus Sweden.
Personal life
Kolbeinn is the son of retired international footballer Finnur Kolbeinsson.
