Þorsteinn graduated from the Menntaskólinn í Reykjavík gymnasium in 1961 and subsequently received a grant to study at Harvard University.
At Oxford, he studied under Gilbert Ryle and befriended well-known characters from philosophical circles, such as Alfred Jules Ayer.
Þorsteinn received a number of awards and acknowledgements for his works, amongst them the Þórbergur Þórðarson and National Icelandic Literature Prizes in 1997.
He also found inspiration in the continental philosophers, especially from existentialism in the style of Søren Kierkegaard and Martin Heidegger, whose works he translated into Icelandic.
Later on, his focus was on contemporary work by Elizabeth Anscombe, Donald Davidson, Saul Kripke, John Rawls, Philippa Foot, Charles Taylor and Richard Rorty.
Þorsteinn's early works (An Essay on Man in particular) are in the style of 20th century logical positivism—highly suspicious of esoteric Hegelian metaphysics.