Guðni is the son of teacher and journalist Margrét Thorlacius and sports instructor Jóhannes Sæmundsson.

While at MR, he competed in Gettu betur, an Icelandic team quiz show for junior college students.

[2] In 1999, he completed an MSt degree in history at St Antony's College at the University of Oxford.

[1] At the time of his presidential candidacy he worked as a senior lecturer in history at the University of Iceland.

Before his candidacy, he had appeared frequently on live television to provide commentary and historical context in the wake of the publication of the Panama Papers, which created a scandal for Icelandic prime minister Sigmundur Davíð Gunnlaugsson and ultimately led to his ouster.

[13] Guðni took office as President of Iceland on 1 August 2016[14] after winning the most votes in that year's election, 71,356 (39.1%).

[8] Guðni attracted international attention in February 2017 when he jokingly vowed to ban pineapple as a pizza topping.

[25] On 1 January 2024, Guðni announced in his New Year's address to the Icelandic people that he would not stand for election again in 2024.

He left the Catholic Church due to its delayed and muted response to reports of criminal abuses by priests.

He has two brothers, Patrekur (a former Icelandic men's national handball team player, who is coach of the Austria men's national handball team and father of rapper JóiPé)[29] and Jóhannes, who is a systems analyst.

On 1 September 2022, he presented a book about the Icelandic Cod Wars with the United Kingdom, Stund milli stríða.

[37] Guðni also shares ancestry with former US president Donald Trump through their descent from Haakon V of Norway.