Hafþór was born in Reykjavík. His father, Björn, is over 6 ft 0 in (1.83 m) in height and his mother, Ragnheiður, is of similar stature. Hafþór's grandfather, Reynir, is nearly as tall as Hafþór 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".