When Þorsteinn Sigurðsson was born about 1525, in Möðruvellir, Eyjafjarðarsýsla, Northeastern Region, Iceland, his father, Sigurður Þorsteinsson, was 23 and his mother, Guðrún Finnbogadóttir, was 17. He had at least 2 sons. He died in Grímsey, Eyjafjarðarsýsla, Northeastern Region, Iceland.