Umberto's father urged him to become a Lawyer, but he entered the University of Turin to take up medieval philosophy and literature, writing his thesis on Thomas Aquinas and earning a Laurea degree in philosophy in 1954. During his university studies, Eco stopped believing in God and left the Catholic Church. After that, Eco worked as a cultural Editor for the state broadcasting station Radiotelevisione Italiana (RAI) and lectured at the University of Turin (1956–1964). A group of avant-garde artists, Painters, Musicians and Writers, whom he had befriended at RAI (Gruppo 63), became an important and influential component in Eco's writing career. This was especially true after the publication of his first book in 1956, Il problema estetico in San Tommaso, which was an extension of his Laurea thesis. This also marked the beginning of his lecturing career at his alma mater.

In 1959 Eco published his second book, Sviluppo dell'estetica medievale (The Development of Medieval Aesthetics), on medieval philosophy. After 18 months' military Service in the Italian Army, he left RAI in 1959 to become the senior non-fiction Editor of the Bompiani publishing house in Milan, a position he occupied until 1975.

Eco's short 1961 essay "Fenomenologia di Mike Bongiorno" ("Phenomenology of Mike Bongiorno", on the most popular quiz show host in Italy) received much notoriety among the general public and has drawn endless questions by journalists at every public appearance by Eco; the essay was later included in the collection Diario minimo (1963). His book Apocalittici e integrati (1964) analyzes the phenomenon of mass communication from a sociological perspective.

Eco began seriously developing his ideas on the "open" text and on semiotics, writing many essays on these subjects, and in 1962 he published Opera aperta (translated into English as "The Open Work"). In it, Eco argued that literary texts are fields of meaning, rather than strings of meaning; and that they are understood as open, internally dynamic and psychologically engaged fields. Literature which limits one's potential understanding to a single, unequivocal line, the closed text, remains the least rewarding, while texts that are the most active between mind, society and life (open texts) are the liveliest and best—although valuation terminology was not his primary focus. Eco came to these positions through study of language and from semiotics, rather than from psychology or historical analysis (as did theorists such as Wolfgang Iser, on the one hand, and Hans Robert Jauss, on the other).

In 1967 he gave the influential lecture "Towards a Semiological Guerrilla Warfare", which coined the influential term "semiological guerrilla", and influenced the theorization of guerrilla tactics against mainstream mass media culture, such as guerrilla television and culture jamming. Among the expressions used in the essay are "communications guerrilla warfare" and "cultural guerrilla". The essay was later included in Eco's book Faith in Fakes.

Eco founded and developed one of the most important approaches in contemporary semiotics, usually referred to as interpretative semiotics. The main books in which he elaborates his theory are La struttura assente (1968; literally: The Absent Structure), A Theory of Semiotics (1975), The Role of the Reader (1979), Semiotics and Philosophy of Language (1984), The Limits of Interpretation (1990), Kant and the Platypus (1997), and From the Tree to the Labyrinth: Historical Studies on the Sign and Interpretation (2014).

Eco employed his education as a medievalist in his first novel The Name of the Rose (1980), a historical mystery set in a 14th-century monastery. Franciscan friar william of Baskerville, aided by his assistant Adso, a Benedictine novice, investigates a series of murders at a monastery that is to host an important religious debate. The novel contains many direct or indirect metatextual references to other sources, requiring the detective work of the reader to 'solve'. The title is unexplained in the book. As a symbol, the rose is ubiquitous enough not to confer any single meaning. There is a tribute to Jorge Luis Borges, a major influence on Eco, in the blind monk and librarian Jorge of Burgos: Borges, like the character Jorge, lived a celibate life consecrated to his passion for books, and also went blind in later life. william of Baskerville is a logically-minded Englishman who is a friar and a detective, and his name evokes both william of Ockham and Sherlock Holmes (by way of The Hound of the Baskervilles); several passages describing him are strongly reminiscent of Sir Arthur Conan Doyle's descriptions of Holmes. The underlying mystery of the murder is borrowed from the "Arabian Nights". The Name of the Rose was later made into a motion picture starring Sean Connery, F. Murray Abraham, Christian Slater and Ron Perlman, which follows the plot, though not the philosophical and historical themes, of the novel.

In Foucault's Pendulum (1988), three under-employed editors who work for a minor publishing house decide to amuse themselves by inventing a conspiracy theory. Their conspiracy, which they call "The Plan", is about an immense and intricate plot to take over the world by a secret order descended from the Knights Templar. As the game goes on, the three slowly become obsessed with the details of this plan. The game turns dangerous when outsiders learn of The Plan, and believe that the men have really discovered the secret to regaining the lost treasure of the Templars.

He divided his time between an apartment in Milan and a vacation house near Urbino. He had a 30,000 volume library in the former and a 20,000 volume library in the latter. He was a visiting professor at Columbia University several times in the 1980s and 1990s. In 1992–1993 Eco was the Norton professor at Harvard University. On 8 May 1993, Eco received an honorary Doctor of Humane Letters (D.H.L.) from Indiana University Bloomington in recognition of his over fifteen-year association with the university's Research Center for Language and Semiotic Studies. Six books that were authored, co-authored, or co-edited by Eco were published by the Indiana University Press. In 1996, he was appointed Honorary Doctor of Philosophy of the University of Tartu. He frequently collaborated with his friend Thomas Sebeok, semiotician and Distinguished Professor of Linguistics at Indiana University. He became Satrap of the Collège de 'Pataphysique in 2001. On 23 May 2002, Eco received an honorary Doctor of Letters (D.Litt.) from Rutgers University in New Brunswick, New Jersey. In 2009, the University of Belgrade in Serbia awarded him an honorary doctorate. Eco was a member of the Italian skeptic organization Comitato Italiano per il Controllo delle Affermazioni sulle Pseudoscienze (Italian Committee for the Investigation of Claims of the Pseudosciences) CICAP.

The Island of the Day Before (1994) was Eco's third novel. The book, set in the seventeenth century, is about a man marooned on a ship within sight of an island which he believes is on the other side of the international date-line. The main character is trapped by his inability to swim and instead spends the bulk of the book reminiscing on his life and the adventures that brought him to be marooned.

Baudolino was published in 2000. Baudolino is a knight who saves the Byzantine Historian Niketas Choniates during the sack of Constantinople in the Fourth Crusade. Claiming to be an accomplished liar, he confides his history, from his childhood as a peasant lad endowed with a vivid imagination, through his role as adopted son of Emperor Frederick Barbarossa, to his mission to visit the mythical realm of Prester John. Throughout his retelling, Baudolino brags of his ability to swindle and tell tall tales, leaving the Historian (and the reader) unsure of just how much of his story was a lie.

The Mysterious Flame of Queen Loana (2005) is about Giambattista Bodoni, an old bookseller specializing in antiques who emerges from a coma with only some memories to recover his past. Bodoni is pressed to make a very difficult choice, one between his past and his Future. He must either abandon his past to live his Future or regain his past and sacrifice his Future.

The Prague Cemetery, Eco's sixth novel, was published in 2010. It is the story of a secret agent who "weaves plots, conspiracies, intrigues and attacks, and helps determine the historical and political fate of the European Continent". The book is a narrative of the rise of Modern-day antisemitism, by way of the Dreyfus affair, The Protocols of the Elders of Zion and other important 19th century events which gave rise to hatred and hostility toward the Jewish people.

In 2012, Eco and Jean-Claude Carrière published a book of conversations on the Future of information carriers. Eco has criticized social networks saying for Example that "Social media gives legions of idiots the right to speak when they once only spoke at a bar after a glass of wine, without harming the community [...] but now they have the same right to speak as a Nobel Prize winner. It's the invasion of the idiots."

Numero Zero was published in 2015. Set in 1992 and narrated by Colonna, a hack Journalist working on a Milan newspaper, it offers a satire of Italy's kickback and bribery culture as well as, among many things, the legacy of Fascism.

Eco died at his Milanese home of pancreatic cancer, from which he had been suffering for two years, on the night of 19 February 2016. At the time of his death at the age of 84, he was a professor emeritus at the University of Bologna, a position that he had held since 2008.