Born in Reykjavík, Hafþór began his basketball career in 2004 with Division I team Breiðablik, moving to FSu in 2005. He transferred to Premier League side KR in 2006 before returning to FSu in 2007. He helped FSu to promotion from Division I to the Premier League but his career was cut short in 2008 due to a recurrent ankle injury. He subsequently began his strongman career.
Hafþór won the Strongest Man in Iceland event in 2010, and Iceland's Strongest Man in 2011. He won Europe's Strongest Man in 2014, a feat he repeated in 2015, 2017, 2018, and 2019. He won gold at the Arnold Strongman Classic in 2018, 2019 and 2020. He first competed in the World's Strongest Man in 2011, placing sixth. He won three bronze and three silver medals in his next six attempts before being crowned champion in 2018.
On 2 May 2020, Hafþór deadlifted a record-setting 501 kilograms (1,105 lb) under strongman rules at his gym in Iceland and consequently has the world record deadlift.
Hafþór began his athletic career as a basketball player. At 2.06 m (6 ft 9 in), he started his senior team career with Division I club Breiðablik in 2004, playing as a center. In 2005 he joined FSu Selfoss, also in the Icelandic Division I. After 10 games it was discovered that he had been playing with a broken bone in his ankle and would require surgery. After recovering from surgery, Hafþór moved to Úrvalsdeild powerhouse KR in 2006. He missed most of the 2006–2007 season after a screw in his ankle shattered and had to be fixed by another surgery.
For the 2007–2008 season, he moved back to play for FSu Selfoss. He averaged 6.7 points per game, helping the team achieve promotion to the Úrvalsdeild, but the troublesome ankle forced him to retire from basketball after the season, at the age of 20.
Hafþór met Icelandic strongman Magnús Ver Magnússon at his gym "Jakaból" in 2008, and Magnús said that Hafþór seemed a good prospect as a strongman. Hafþór won several strongman contests in Iceland in 2010, including Strongest Man in Iceland, Iceland's Strongest Viking, Westfjords Viking, and five of six events at the OK Badur Strongman Championships.
In January 2015, at the World's Strongest Viking competition held in Norway, Hafþór carried a 10-metre-long (33 ft), 650-kilogram (1,430 lb) log for five steps, thus breaking a 1,000-year-old record set by Orm Storolfsson.
On 15 February 2020, Hafþór deadlifted an unofficial world record on the elephant bar with 480 kilograms (1,058 lbs).
On 2 May 2020, Hafþór deadlifted 501 kilograms (1,105 lb) under strongman rules at his gym in Iceland while being refereed by Magnús Ver Magnússon. This achievement was televised live by ESPN. Hafþór surpassed the strongman deadlift record of 500 kilograms (1,102 lb) which was set by Eddie Hall in 2016.
In May 2020, Hafþór stated that he would never return to a Giants Live event or a World's Strongest Man competition, thus statedly ending Hafþór's participation in World's Strongest Man.
Arnold Strongman Classic
On 3 March 2018, Hafþór broke the Elephant Bar deadlift world record with his successful 472 kg (1,041 lb) third attempt, as well as being champion of the Arnold Strongman Classic 2018. The previous Elephant Bar deadlift world record of 468 kg (1,032 lb) was set by Jerry Pritchett at the Arnold Strongman Classic 2017.
Hafþór defended his title at the Arnold Strongman Classic 2019; he finished on 45 points, 4.5 points ahead of runner-up Martins Licis. He also improved on his elephant bar world record, increasing it to 474 kg (1,045 lb) with his second attempt. He tried for 501 kg (1,105 lb) in his third attempt but was unsuccessful.
After successfully defending his crown, Hafþór became only the second person to win three Arnold Strongman Classics in a row (2018, 2019 and 2020) after Zydrunas Savickas.
In a 2017 interview, when asked if he had ever used steroids, Hafþór answered: "'Yes, I have'". He added, "'When you want to be the best, you do whatever it takes'". Hafþór declined to say when he had used steroids or whether his use of the substances was ongoing.
Hafþór played the lead role in the Philadelphia Renaissance Faire during their debut season in 2015. He appeared as "King Thor", the leader of a Viking raiding party intent on capturing the city of Amman.
Family and relationships
Hafþór was born in Reykjavík. His father, Björn, stands at 203 cm (6 ft 8 in), and his mother, Ragnheiður, is also of very tall stature. Hafþór's grandfather, Reynir, is 207 cm (6 ft 9+1⁄2 in) and "just as broad across the chest". Reynir recalled in 2014 that Hafþór worked on the family farm outside Reykjavík as a child and "was always big, even when he wasn't tall".
Hafþór has a daughter with his former girlfriend Thelma Björk Steimann. Until early 2017, he was in a relationship with Andrea Sif Jónsdóttir whom he met at a gym. They lived together in Reykjavik. In an online video interview by Mulligan Brothers published 21 December 2019, Hafþór lamented not having seen his daughter in three years.
Since late 2017, Hafþór had been dating Kelsey Henson, a Canadian waitress. The two met when Hafþór was in Alberta, Canada for a strongman competition and visited the bar where Henson worked. The couple garnered attention for the difference in their respective heights, as Henson is 157 cm (5 ft 2 in). On 21 October 2018, Hafþór announced that he and Henson had married. On 26 September 2020, Hafþór and Henson welcomed their first child together, a baby boy.
Hafþór has been accused of domestic violence by several ex girlfriends, causing an injury to one woman that led to her hospitalization. The woman pressed charges but the case did not go further than that. Hafþór has since reportedly charged the woman with slander. 
On 8 June 2017, the police was called to Hafþór's home as his girlfriend was seen escaping out of a window. The police had been called to his home three times that year because of domestic disputes. Each time, his girlfriend appeared battered and bruised, according to the police. Three more of Hafþór's ex-girlfriends since came forward and accused him of domestic violence and neighbours have reported police arriving to his residence after hearing a woman screaming. Hafþór and his lawyer threatened any reporting of domestic abuse allegations with legal action.  Subsequently, Thelma Björk Steimann, the mother of one of his children, came forward for the first time in an extensive interview, saying "she feared for her life" while they were together.