Björnsson started a professional basketball career in Iceland but it was cut short when he injured his knee forcing an early retirement from the sport. Soon after in 2008, he met Magnús Ver Magnússon, a four time World's Strongest Man winner who said Hafþór seemed a good prospect to be a strongman. In 2010, he came third in Iceland's Strongest Man competition and has won every year since then. He has also won Europe's Strongest Man competition in 2014 and has appeared in four World's Strongest Man finals through 2011-2014, finishing in sixth, third, third, and second respectively only narrowly missing the title in 2014 to Lithuanian four time World's Strongest Man winner, and friend of Hafþór's, Žydrūnas Savickas. Savickas was the previous reigning champion as Europe's Strongest Man since 2010, but did not compete in 2014 (Savickas won in 2010, 2012, and 2013, but the Europe's Strongest Man competition was on hiatus in 2011 and no champion was named that year).