Born in Hafnarfjörður, Dagur spent time in the Fylkir youth ranks before joining the Haukar academy in 2013. He made his senior debut with Haukar on 12 September 2016, starting and playing the full 90 minutes of a 1–1 draw with HK in the 2016 1. deild karla. He made six appearances for the club before leaving to join the youth ranks of Belgian Pro League club Gent. He returned to Icelandic football with Keflavík in 2018, playing in the Úrvalsdeild. In the first half of 2019 he was loaned to Mjøndalen in Norway's top flight, making one league appearance and a further four in the cup before joining Mjøndalen permanently in the summer. He was immediately loaned out for the remainder of the 2019 season to third tier team Kvik Halden where he played regularly, scoring five goals in 12 league appearances as the team reached the promotion playoffs. In 2021, Dagur returned to Iceland on loan from Mjøndalen, joining Fylkir for the 2021 Úrvalsdeild season. Ahead of the 2022 season he made a permanent transfer to Breiðablik.
Dagur received his first senior international call-up for Iceland in November 2022 for a pair of friendlies against Saudi Arabia and South Korea. Played outside of a FIFA window, the squad was a chance for manager Arnar Viðarsson to work with domestic players and those from other leagues not active in November. He made his debut on 6 November 2022, starting and playing the full 90 minutes in a 1–0 defeat to Saudi Arabia in Abu Dhabi.[5] After missing the 2022 Baltic Cup, he returned to the squad in January 2023 for friendlies against Estonia and Sweden, starting both.[4]
Personal life
Dagur is the son of former international footballer Þórhallur Dan Jóhannsson.[6] On 1 December 2024, Dagur and his partner Friðrika Arnardóttir announced on Instagram that they were having a child together.[7]
