Julio Cortázar was born on August 26, 1914, in Ixelles, a municipality of Brussels, Belgium. According to biographer Miguel Herráez, his parents, Julio José Cortázar and María Herminia Descotte, were Argentine citizens, and his father was attached to the Argentine diplomatic Service in Belgium.

At the time of Cortázar's birth Belgium was occupied by the German troops of Kaiser Wilhelm II. After German troops arrived in Belgium, Cortázar and his family moved to Zürich where María Herminia's parents, Victoria Gabel and Louis Descotte (a French National), were waiting in neutral territory. The family group spent the next two years in Switzerland, first in Zürich, then Geneva, before moving for a short period to Barcelona. The Cortázars settled outside of Buenos Aires by the end of 1919.

Cortázar obtained a qualification as an elementary school Teacher at the age of 18. He would later pursue higher education in philosophy and languages at the University of Buenos Aires, but left for financial reasons without receiving a degree. According to biographer Montes-Bradley, Cortázar taught in at least two high schools in Buenos Aires Province, one in the city of Chivilcoy, the other in Bolivar. In 1938, using the pseudonym of Julio Denis, he self-published a volume of sonnets, Presencia, which he later repudiated, saying in a 1977 interview for Spanish television that publishing it was his only transgression to the principle of not publishing any books until he was convinced that what was written in them was what he meant to say. In 1944 he became professor of French literature at the National University of Cuyo in Mendoza, but he resigned the position in June 1946 due to political pressure from Peronists. He subsequently worked as a translator and as Director of the Cámara Argentina del Libro, a trade organization. In 1949 he published a play, Los Reyes (The Kings), based on the myth of Theseus and the Minotaur. In 1980, Cortázar delivered eight lectures at the University of California, Berkeley.

Cortázar wrote numerous short stories, collected in such volumes as Bestiario (1951), Final del juego (1956), and Las armas secretas (1959). In 1967, English translations by Paul Blackburn of stories selected from these volumes were published by Pantheon Books as End of the Game and Other Stories; it was later re-titled Blow-up and Other Stories. Cortázar published four novels during his lifetime: Los premios (The Winners, 1960), Hopscotch (Rayuela, 1963), 62: A Model Kit (62 Modelo para Armar, 1968), and Libro de Manuel (A Manual for Manuel, 1973). Except for Los premios, which was translated by Elaine Kerrigan, these novels have been translated into English by Gregory Rabassa. Two other novels, El examen and Divertimento, though written before 1960, only appeared posthumously.

Cortázar had three long-term romantic relationships with women. The first was with Aurora Bernárdez, an Argentine translator, whom he married in 1953. They separated in 1968 when he became involved with the Lithuanian Writer, Editor, translator, and filmmaker Ugnė Karvelis, whom he never formally married, and who reportedly stimulated Cortázar's interest in politics, although his political sensibilities had already been awakened by a visit to Cuba in 1963, the first of multiple trips that he would make to that country throughout the remainder of his life. He later married Canadian Writer Carol Dunlop. After Dunlop's death in 1982, Aurora Bernárdez accompanied Cortázar during his final illness and, in accordance with his longstanding wishes, inherited the rights to all his works.

Cortázar also published poetry, drama, and various works of non-fiction. In the 1960s, working with the Artist José Silva, he created two almanac-books or libros-almanaque, La vuelta al día en ochenta mundos and Último Round, which combined various texts written by Cortázar with a photographs, engravings, and other illustrations, in the manner of the almanaques del mensajero that had been widely circulated in rural Argentina during his childhood. One of his last works was a collaboration with Carol Dunlop, The Autonauts of the Cosmoroute, which relates, partly in mock-heroic style, the couple's extended expedition along the autoroute from Paris to Marseille in a Volkswagen camper nicknamed Fafner. As a translator, he completed Spanish-language renderings of Robinson Crusoe, Marguerite Yourcenar's novel Mémoires d'Hadrien, and the complete prose works of Edgar Allan Poe.

Cortázar's father left when Julio was six, and the family had no further contact with him. Cortázar spent most of his childhood in Banfield, a suburb south of Buenos Aires, with his mother and younger sister. The home in Banfield, with its back yard, was a source of inspiration for some of his stories. Despite this, in a letter to Graciela M. de Solá on December 4, 1963, he described this period of his life as "full of servitude, excessive touchiness, terrible and frequent sadness." He was a sickly child and spent much of his childhood in bed reading. His mother, who spoke several languages and was a great reader herself, introduced her son to the works of Jules Verne, whom Cortázar admired for the rest of his life. In the magazine Plural (issue 44, Mexico City, May 1975) he wrote: "I spent my childhood in a haze full of goblins and elves, with a sense of space and time that was different from everybody else's."

Michelangelo Antonioni's film Blowup (1966) was inspired by Cortázar's story "Las babas del Diablo," which in turn was based on a photograph taken by Chilean Photographer Sergio Larraín during a shoot outside of Notre Dame Cathedral in Paris. Cortázar's story "La autopista del sur" ("The Southern Thruway") influenced another film of the 1960s, Jean-Luc Godard's Week End (1967). The filmmaker Manuel Antín has directed three films based on Cortázar stories, Cartas de mamá, Circe, and Intimidad de los parques.

He died in Paris in 1984 and is interred in the Cimetière de Montparnasse. The cause of his death was reported to be leukemia though some sources state that he died from AIDS as a result of receiving a blood transfusion.

Puerto Rican Novelist Giannina Braschi used Cortázar's story "Las babas del diablo" as a springboard for the chapter called "Blow-up" in her bilingual novel Yo-Yo Boing! (1998), which features scenes with Cortázar's characters La Maga and Rocamadour. Cortázar is mentioned and spoken highly of in Rabih Alameddine's 1998 novel, Koolaids: The Art of War.