Alekos Fassianos was born in Athens in 1935 to a musician father and ancient Greek teacher mother. He studied violin at the National Conservatory and painting with Yannis Moralis at the School of Fine Arts in Athens from 1956 to 1960. In 1960 he received a scholarship from the French government and travelled to Paris to study lithography at the École des Beaux-Arts. Following the rise of the Colonels' Dictatorship in Greece, Fassianos moved permanently to Paris.
In 1969, the dealer Paul Facchetti discovered his painting and started a collaboration with Fassianos, which led him to exhibit together with the likes of Jean Paul Riopelle and Georges Mathieu. The solo exhibitions organised by Facchetti were fundamental for Fassianos, as they helped him to focus on a distinctive and highly personal language. In the 1970s, the artist was represented by the Parisian gallery of Alexander Iolas, together with artists such as Max Ernst, Magritte, de Chirico and Martial Raysse. During his long career, Fassianos exhibited in Malmö and Tokyo, as well as in Zurich, Berlin, Milan and New York.
Besides being a painter, Fassianos was a writer, poet, ceramist, set designer and architect. He worked as a costume designer, creating the costumes and sets for numerous plays, including Franz Kafka's America at the National Theatre in Athens and Euripides' Helen (1980). He also designed the set design for Jean-Paul Sartre's No Exit. Fassianos also illustrated numerous literary works by writer, thinker and poet friends such as Constantin Cavafy, Miltos Sachtouris, Louis Aragon, Hamid Fouladvind, OdysseasElytis, Guillaume Apollinaire, Jacques Lacarriere, Paul Valéry, Arrabal and Jean-Marie Drot. He created limited editions and publications with Bruno Roy, the publisher of Fata Morgana and André Biren.