[1] From 2005 to 2010, Jóhannesson was the Ambassador to the Netherlands, Luxembourg, Morocco, Switzerland and the EU.
[3][4][5] Between 2009 and 2013, Jóhannesson was the Chief Negotiator in Iceland's accession talks with the European Union.
He presented his Letters of Credence to Queen Elizabeth II on 14 December 2017.
[9] Jóhannesson was already in August 2020 appointed the Ambassador to Japan and originally would take the post at the turn of the year between 2020 and 2021.
[11][12] Jóhannesson is married to Halldóra Hermannsdóttir and they have three children, including Einar Stefánsson, musician from Vök and Hatari.