Kristján Jóhannesson was born in Reykjavík in 1992, where he studied voice from the year 2008. His teachers were Sigrún Hjálmtýsdóttir, Kristján Jóhannsson and Keith Reed. In 2014 he moved to Vienna to further his studies with Uta Schwabe. During his studies he took part in productions of the Summer Academy of the Vienna Philharmonic and Neue Oper Wien, in Hof in Akureyri with the North Iceland Symphony Orchestra and with the Icelandic Opera. In 2017 Kristján commenced his career at Theater an der Wien and Kammeroper in Vienna. His productions there include Der Ring des Nibelung and Tristan und Isolde by Richard Wagner, Salome by Richard Strauss, Don Carlos by Giuseppe Verdi, Faust by Charles Gounod and The Maid of Orleans by Piotr Tchaikovsky. Kristján then moved to the Theatre of St. Gallen in Switzerland in 2021. Notable projects there include the Requiem of Verdi and his Ernani, Elektra by Richard Strauss, Guillaume Tell by Gioachino Rossini, Die Fledermaus by Johann Strauss II and Die Zauberflöte by Mozart. He has also appeared at the Festival of Aix-en-Provence.

He has participated in numerous masterclasses with singers such as Elly Ameling, Andreas Schmidt, Robert Holl, Thomas Hampson, Angelika Kirchschlager and Adrian Eröd, and with pianists Helmut Deutsch, Julius Drake, Wolfram Rieger and Roger Vignoles.