In 1913, his 14-year-old brother Renato died suddenly during a football game; Andrade left the Conservatory to stay at Araraquara, where his family had a farm. When he returned, his piano playing was afflicted intermittently by trembling of his hands. Although he ultimately did receive a degree in piano, he gave no concerts and began studying singing and music theory with an eye toward becoming a professor of music. At the same time, he began writing more seriously. In 1917, the year of his graduation, he published his first book of poems, Há uma Gota de Sangue em Cada Poema (There is a drop of blood in each poem), under the pseudonym Mário Sobral. The book contains hints of Andrade's growing sense of a distinctive Brazilian identity, but it does so within the context of a poetry that (like most Brazilian poetry of the period) is strongly indebted to earlier European—particularly French—literature.

Throughout the 1920s Andrade continued traveling in Brazil, studying the culture and folklore of the interior. He began to formulate a sophisticated theory of the social dimensions of folk music, which is at once nationalistic and deeply personal. Andrade's explicit subject was the relationship between "artistic" music and the music of the street and countryside, including both Afro-Brazilian and Amerindian styles. The work was controversial for its formal discussions of dance music and folk music; those controversies were compounded by Andrade's style, which was at once poetic (Luper calls it "Joycean") and polemical.

His first book does not seem to have had an enormous impact, and Andrade broadened the scope of his writing. He left São Paulo for the countryside, and began an activity that would continue for the rest of his life: the meticulous documentation of the history, people, culture, and particularly music of the Brazilian interior, both in the state of São Paulo and in the wilder areas to the northeast. He published essays in São Paulo magazines, accompanied occasionally by his own photographs, but primarily he accumulated massive amounts of information about Brazilian life and folklore. Between these trips, Andrade taught piano at the Conservatory, and became one of its professors in 1921.

In 1922, while preparing Paulicéia Desvairada for publication, Andrade collaborated with Malfatti and Oswald de Andrade in creating a single event that would introduce their work to the wider public: the Semana de Arte Moderna (Week of Modern Art). The Semana included exhibitions of paintings by Malfatti and other artists, readings, and lectures on art, music, and literature. Andrade was the chief organizer and the central figure in the event, which was greeted with skepticism but was well-attended. He gave lectures on both the principles of modernism and his work in Brazilian folk music, and read his "Extremely Interesting Preface." As the climactic event of the Semana, he read from Paulicéia Desvairada. The poems' use of free verse and colloquial São Paulo expressions, though related to European modernist poems of the same period, were entirely new to Brazilians. The reading was accompanied by persistent jeers, but Andrade persevered, and later discovered that a large part of the audience found it transformative. It has been cited frequently as the seminal event in modern Brazilian literature.

Though Andrade continued taking photographs throughout his career, these images from the 20s comprise the bulk of his notable work, and the 1927 series in particular. He was particularly interested in the capacity of photographs to capture or restate the past, a power he saw as highly personal. In the late 1930s, he wrote:

At the same time, Andrade was developing an extensive familiarity with the dialects and cultures of large parts of Brazil. He started to apply to prose fiction the speech-patterned technique he had developed in writing the poems of Hallucinated city. He wrote two novels during this period using these techniques: the first, Love, Intransitive Verb, was largely a formal experiment.; the second, written shortly after and published in 1928, was Macunaíma, a novel about a man ("The hero without a character" is the subtitle of the novel) from an indigenous tribe who comes to São Paulo, learns its languages—both of them, the novel says: Portuguese and Brazilian—and returns. The style of the novel is composite, mixing vivid descriptions of both jungle and city with abrupt turns toward fantasy, the style that would later be called magical realism. Linguistically, too, the novel is composite; as the rural hero comes into contact with his urban environment, the novel reflects the meeting of languages. Relying heavily on the primitivism that Andrade learned from the European modernists, the novel lingers over possible indigenous cannibalism even as it explores Macunaíma's immersion in urban life. Critic Kimberle S. López has argued that cannibalism is the novel's driving thematic force: the eating of cultures by other cultures.

Other critics have argued for similar analogues between Andrade's sexuality and Macunaíma's complex status. Though Andrade was not openly gay, and there is no direct evidence of his sexual practices, many of Andrade's friends have reported after his death that he was clearly interested in men (the subject is only reluctantly discussed in Brazil). It was over a pseudonymous accusation of effeminacy that Andrade broke with Oswald de Andrade in 1929. Macunaíma prefers women, but his constant state of belonging and not belonging is associated with sex. The character is sexually precocious, starting his romantic adventures at the age of six, and his particular form of eroticism seems always to lead to destruction of one kind or another.

Andrade was not directly affected by the Revolution of 1930, in which Getúlio Vargas seized power and became dictator, but he belonged to the landed class the Revolution was designed to displace, and his employment prospects declined under the Vargas regime. He was able to remain at the Conservatory, where he was now Chair of History of Music and Aesthetics. With this title he became a de facto national authority on the history of music, and his research turned from the personal bent of his 1920s work to textbooks and chronologies. He continued to document rural folk music, and during the 1930s made an enormous collection of recordings of the songs and other forms of music of the interior. The recordings were exhaustive, with a selection based on comprehensiveness rather than an aesthetic judgment, and including context, related folktalkes, and other non-musical sound. Andrade's techniques were influential in the development of ethnomusicology in Brazil and predate similar work done elsewhere, including the well-known recordings of Alan Lomax. He is credited with coining the word "popularesque," which he defined as imitations of Brazilian folk music by erudite urban Musicians ("erudite" is generally a deprecation in Andrade's vocabulary). The word continues to have currency in discussion of Brazilian music as both a scholarly and nationalist category.

In 1935, during an unstable period in Vargas's government, Andrade and Writer and archaeologist Paulo Duarte, who had for many years desired to promote cultural research and activity in the city through a municipal agency, were able to create a unified São Paulo Department of Culture (Departamento de Cultura e Recreação da Prefeitura Municipal de São Paulo). Andrade was named founding Director. The Department of Culture had a broad purview, overseeing cultural and demographic research, the construction of parks and playgrounds, and a considerable publishing wing. Andrade approached the position with characteristic ambition, using it to expand his work in folklore and folk music while organizing myriad performances, lectures, and expositions. He moved his collection of recordings to the Department, and expanding and enhancing it became one of the Department's chief functions, overseen by Andrade's former student, Oneyda Alvarenga. The collection, called the Discoteca Municipal, was "probably the largest and best-organized in the entire hemisphere."

Andrade's position at the Department of Culture was abruptly revoked in 1937, when Vargas returned to power and Duarte was exiled. In 1938 Andrade moved to Rio de Janeiro to take up a post at the Universidade Federal do Rio de Janeiro. While there he directed the Congresso da Língua Nacional Cantada (Congress of National Musical Language), a major folklore and folk music conference. He returned to São Paulo in 1941, where he worked on a collected edition of his poetry.

Andrade died at his home in São Paulo of a heart attack on February 25, 1945, at the age of 52. Because of his tenuous relationship with the Vargas regime, the initial official reaction to his career was muted. However, the publication of his Complete Poems in 1955 (the year after Vargas's death) signalled the start of Andrade's canonization as one of the cultural heroes of Brazil. On February 15, 1960, the municipal library of São Paulo was renamed Biblioteca Mário de Andrade.

Inevitably, Macunaíma's polemicism and sheer strangeness have become less obvious as it has grown ensconced in mainstream Brazilian culture and education. Once regarded by academic critics as an awkwardly constructed work of more historical than literary importance, the novel has come to be recognized as a modernist masterpiece whose difficulties are part of its aesthetic. Andrade is a national cultural icon; his face has appeared on the Brazilian currency. A film of Macunaíma was made in 1969, by Brazilian Director Joaquim Pedro de Andrade, updating Andrade's story to the 1960s and shifting it to Rio de Janeiro; the film was rereleased internationally in 2009.

Even as Macunaíma changed the nature of Brazilian literature in an instant—Albuquerque calls it "the cornerstone text of Brazilian Modernism"—the inner conflict in the novel was a strong part of its influence. Modernismo, as Andrade depicted it, was formally tied to the innovations of recent European literature and based on the productive meeting of cultural forces in Brazil's diverse population; but it was fiercely nationalistic, based in large part on distinguishing Brazil's culture from the world and on documenting the damage caused by the lingering effects of colonial rule. At the same time, the complex inner life of its hero suggests themes little explored in earlier Brazilian literature, which critics have taken to refer back to Andrade himself. While Macunaíma is not autobiographical in the strict sense, it clearly reflects and refracts Andrade's own life. Andrade was a mulatto; his parents were landowners but were in no sense a part of Brazil's Portuguese pseudo-aristocracy. Some critics have paralleled Andrade's race and family background to the interaction between categories of his character Macunaíma. Macunaíma's body itself is a composite: his skin is darker than that of his fellow tribesmen, and at one point in the novel, he has an adult's body and a child's head. He himself is a wanderer, never belonging to any one place.

At the same time, Andrade was refining his theory of music. He attempted to pull together his research into a general theory. Concerned as always with Modernismo's need to break from the past, he formulated a distinction between the classical music of 18th- and 19th-century Europe, and what he called the music of the Future, which would be based simultaneously on modernist breakdowns of musical form and on an understanding of folk and popular music. The music of the past, he said, was conceived in terms of space: whether counterpoint, with its multiple voices arranged in vertical alignment, or the symphonic forms, in which the dominant voice is typically projected on top of a complex accompaniment. Future music would be arranged in time rather than space: "moment by moment" (in Luper's translation). This temporal music would be inspired not by "contemplative remembrance", but by the deep longing or Desire expressed by the Portuguese word saudade.