Matthías worked for the Morgunblaðið newspaper for most of his working life, as a journalist from 1951–1959 when he became an editor, a position he held until his retirement in 2001. In his long writing career, Matthías Johannessen has published a number of poetry books, plays, essays, interview books, biographies and translations. He has also edited a number of books, written prologues for various works and published numerous articles in magazines and newspapers. He is well known for his many interviews with known and unknown people, among them Halldór Laxness, the poet Tómas Guðmundsson and composer Páll Ísólfsson. His first collection of poetry, Borgin hló (The City Laughed) was published in 1958 and gained immediate attention for its free form. His poetry has been translated to many languages and he has received various awards for his work. Three of his books have been nominated for the Nordic Council's Literary Award.