Born in Reykjavík, Iceland, Hjörtur began his career at Fylkir, based in the Icelandic top division. After going through the ranks for the side, Hermannsson was involved in the first team, where he appeared in the first two matches of the season as an unused substitute bench. Weeks later, on 6 June 2011, Hermannsson made his Fylkir debut, where he came on as a late substitute, in a 3–1 loss against Stjarnan. Soon after, Hermannsson became a first team regular and then scored his first goal for the club, as well as his first professional goal, in a 2–1 win over Víkingur. Hermannsson went on to make nine appearance in his first season for the side. Despite moving to PSV Eindhovenin the summer, Hjörtur then made three appearances for the side, adding his tally 12 games for Fylkir, with his first appearance came on 15 May 2012 against ÍA, which saw them lose 1–0.
PSV
After the Úrvalsdeild season came to an end, Hjörtur then went on a trial at Eredivisie side PSV Eindhoven on 29 October 2011. After impressing at the trial, Hjörtur signed a three-year deal with PSV, where he will join the club on 1 July 2012. After years of development at the academy, Hjörtur made his professional debut as Jong PSV player in the Eerste Divisie on 18 August 2013 against FC Oss in a 4–0 home win. He replaced Jorrit Hendrix after 61 minutes. He made 25 league appearances during the 2013–14 season after a knee injury ended his season. In the 2014–15 season, Hjörtur continued to remain in the Jong PSV starting eleven despite suffering setback of injuries, which restricted him to 16 appearances. During the season, Hjörtur was called up to the senior team ahead of the match against NAC Breda, but did not play, as PSV went on to win 2–0 on 3 February 2015. Ahead of the 2015–16 season, Hjörtur signed a contract with the club, keeping him until 2018. By the first half of the season, Hjörtur made 16 appearances for the side and it was announced on 16 February 2016, Hjörtur was loaned out to Allsvenskan club Göteborg until the summer. However, at the start of the season, Hjörtur suffered a hip while on international duty and was sidelined for weeks. After recovering, he then returned from injury and made his Göteborg debut on 2 May 2016, where he made his first start, in a 6–2 win over Gefle. From that moment on, Hjörtur became a first team regular for the side throughout May and June until his loan spell expired in the summer.
Brøndby IF
After his loan spell at Göteborg came to an end, Hjörtur joined Danish side Brøndby for €300,000 and viewed as the replacement for Daniel Agger, who retired at the end of the season. After missing out in the opening game of the season, Hjörtur made his Brøndby debut on 31 July 2016, where he made his first start and played the whole game, in a 2–2 draw against Horsens. Weeks later after making his debut, Hjörtur then scored his first goal for the club on 14 August 2016, in a 4–0 win over SønderjyskE. From that moment on, Hjörtur became a first team regular for the club.
International career
While at the duty for Iceland U17 and Iceland U19 for the past three years, Hjörtur was named as one of the promising players by UEFA ahead of the UEFA European Under-17 Championship in Slovakia and scored once during the campaign against France U17. Following the end of the tournament, Hjörturwas named Team of the Tournament. The following year, Hjörtur was called up by Iceland U21for the first time and made his Iceland U21 debut on 6 February 2013, where he played the whole game despite losing 3–0 against Wales U21. It wasn't until on 5 March 2014 when Hjörtur scored his first Iceland U21 goal, in a 3–2 loss against Kazakhstan U21. A year later, Hjörtur scored again on 5 September 2015, in a 3–2 win over France U21. In October 2016, Hjörtur captained Iceland U21 for the first time, which saw them win 2–0 against Scotland U21. Hjörtur got his first call up to the senior Iceland squad for friendlies against Poland and Slovakia in November 2015. It wasn't until on 31 January 2016 when he made his senior team debut, coming on as a substitute in the second half, in a 3–2 loss against United States. He was selected for UEFA Euro 2016. However, Hjörtur did not make any appearance for Iceland throughout the UEFA Euro 2016, as they reached the quarter-final and lost to France. Despite making no appearance, Hjörtur stated that being called up for the UEFA Euro 2016 was the best day of his life.
