Guðni is the son of teacher and journalist Margrét Thorlacius and sports instructor Jóhannes Sæmundsson [REF].
Guðni graduated from Menntaskólinn í Reykjavík in 1987 and earned a bachelor's degree in history and political science from the University of Warwick in England in 1991 and a Master of Arts in history from the University of Iceland in 1997.