The article explores Merab Mamardashvili`s philosophy of consciousness focusing on its existentialist core. It is shown that Mamardashvili developed a special type of existentialism in which the main role was given to thinking and special state of conscious activity of human being. The similarities and differences in its version of existentialism with other known existentialist theories are also outlined. Unlike the French existentialism questioning the role of ratio, Mamardashvili directly links thinking and existence. However, this opposition must be understood with reservations as Mamardashvili's thinking is not understood as conceptual-discursive processes but as search of meaning, the nature of which according to Mamardashvili is purely existential. In what sense, existentialism of Mamardashvili can be called the doctrine of "conscious life" and special "existential eschatology" [11, p. 406]. Mamardashvili's idea of alternating human existence in two dimensions - living consciousness and its "dead double", in the clarification of which an essential role belongs to the "engagement" of consciousness, namely his interest in "his true position", is analyzed. Mamardavshvili's existentialism notes a special "Russianness" in which he develops a productive role of moral suffering and suggests considering it as the true beginning of "philosophical astonishment", interpreting any initial metaphysical inquiry as necessarily ethical, not epistemological.  It is shown that despite of similarity of Mamardashville's existentialism with existentialism of Sartre, Camus and Heidegger, his reasoning is more close to existentialism of Pascal, as nontrivial combines post-metaphysical existentialism and metaphysical platonism.