Leoš Janáček, son of schoolmaster Jiří (1815–1866) and Amalie (née Grulichová) Janáčková (1819–1884), was born in Hukvaldy, Moravia (then part of the Austrian Empire). He was a gifted child in a family of limited means, and showed an early musical talent in choral singing. His father wanted him to follow the family tradition, and become a Teacher, but deferred to Janáček's obvious musical abilities. In 1865, young Janáček enrolled as a ward of the foundation of the Abbey of St. Thomas in Brno, where he took part in choral singing under Pavel Křížkovský and occasionally played the organ. One of his classmates, František Neumann, later described Janáček as an "excellent Pianist, who played Beethoven symphonies perfectly in a piano duet with a classmate, under Křížkovský's supervision". Křížkovský found him a problematic and wayward student but recommended his entry to the Prague Organ School. Janáček later remembered Křížkovský as a great Conductor and Teacher.

Janáček came from a region characterized by its deeply rooted folk culture, which he explored as a young student under Pavel Křížkovský. His meeting with the folklorist and dialectologist František Bartoš (1837–1906) was decisive in his own development as a folklorist and Composer, and led to their collaborative and systematic collections of folk songs. Janáček became an important collector in his own right, especially of Lachian, Moravian Slovakian, Moravian Wallachian and Slovakian songs. From 1879, his collections included transcribed speech intonations. He was one of the organizers of the Czech-Slavic Folklore Exhibition, an important event in Czech culture at the end of 19th century. From 1905 he was President of the newly instituted Working Committee for Czech National Folksong in Moravia and Silesia, a branch of the Austrian institute Das Volkslied in Österreich (Folksong in Austria), which was established in 1902 by the Viennese publishing house Universal Edition. Janáček was a pioneer and propagator of ethnographic photography in Moravia and Silesia. In October 1909 he acquired an Edison phonograph and became one of the first to use phonographic recording as a folklore research tool. Several of these recording sessions have been preserved, and were reissued in 1998.

In 1874, Janáček became friends with Antonín Dvořák, and began composing in a relatively traditional Romantic style. After his opera Šárka (1887–1888), his style absorbed elements of Moravian and Slovak folk music.

Leoš Janáček's literary legacy represents an important illustration of his life, public work and art between 1875 and 1928. He contributed not only to music journals, but wrote essays, reports, reviews, feuilletons, articles and books. His work in this area comprises around 380 individual items. His writing changed over time, and appeared in many genres. Nevertheless, the critical and theoretical sphere remained his main area of interest.

On his return to Brno he earned a living as a music Teacher, and conducted various amateur choirs. From 1876 he taught music at Brno's Teachers Institute. Among his pupils there was Zdenka Schulzová, daughter of Emilian Schulz, the Institute Director. She was later to be Janáček's wife. In 1876, he also became a piano student of Amálie Wickenhauserová-Nerudová, with whom he co-organized chamber concertos and performed in concerts over the next two years. In February 1876, he was voted choirmaster of the Beseda brněnská Philharmonic Society. Apart from an interruption from 1879 to 1881, he remained its choirmaster and Conductor until 1888.

Janáček created his music theory works, essays and articles over a period of fifty years, from 1877 to 1927. He wrote and edited the Hudební listy journal, and contributed to many specialist music journals, such as Cecílie, Hlídka and Dalibor. He also completed several extensive studies, as Úplná nauka o harmonii (The Complete Harmony Theory), O skladbě souzvukův a jejich spojův (On the Construction of Chords and Their Connections) and Základy hudebního sčasování (Basics of Musical Sčasování). In his essays and books, Janáček examined various musical topics, forms, melody and harmony theories, dyad and triad chords, counterpoint (or "opora", meaning "support") and devoted himself to the study of the mental composition. His theoretical works stress the Czech term "sčasování", Janáček's specific word for rhythm, which has relation to time ("čas" in Czech), and the handling of time in music composition. He distinguished several types of rhythm (sčasovka): "znící" (sounding) – meaning any rhythm, "čítací" (counting) – meaning smaller units measuring the course of rhythm; and "scelovací" (summing) – a long value comprising the length of a rhythmical unit. Janáček used the combination of their mutual action widely in his own works.

Janáček worked tirelessly throughout his life. He led the organ school, was a Professor at the teachers institute and gymnasium in Brno, collected his "speech tunes" and was composing. From an early age, he presented himself as an individualist and his firmly formulated opinions often led to conflict. He unhesitatingly criticized his teachers, who considered him a defiant and anti-authoritarian student. His own students found him strict and uncompromising. Vilém Tauský, one of his pupils, described his encounters with Janáček as somewhat distressing for someone unused to his personality and noted that Janáček's characteristically staccato speech rhythms were reproduced in some of his operatic characters. In 1881, Janáček gave up his leading role with the Beseda brněnská, as a response to criticism, but a rapid decline in Beseda's performance quality led to his recall in 1882.

Janáček's deep and lifelong affection for Russia and Russian culture represents another important element of his musical inspiration. In 1888 he attended the Prague performance of Tchaikovsky's music, and met the older Composer. Janáček profoundly admired Tchaikovsky, and particularly appreciated his highly developed musical thought in connection with the use of Russian folk motifs. Janáček's Russian inspiration is especially apparent in his later chamber, symphonic and operatic output. He closely followed developments in Russian music from his early years, and in 1896, following his first visit of Russia, he founded a Russian Circle in Brno. Janáček read Russian authors in their original language. Their literature offered him an enormous and reliable source of inspiration, though this did not blind him to the problems of Russian society. He was twenty-two years old when he wrote his first composition based on a Russian theme: a melodrama, Death, set to Lermontov's poem. In his later works, he often used literary Models with sharply contoured plots. In 1910 Zhukovsky's Tale of Tsar Berendei inspired him to write the Fairy Tale for Cello and Piano. He composed the rhapsody Taras Bulba (1918) to Gogol's short story, and five years later, in 1923, completed his first string quartet, inspired by Tolstoy's Kreutzer Sonata. Two of his later operas were based on Russian themes: Káťa Kabanová, composed in 1921 to Alexander Ostrovsky's play, The Storm: and his last work, From the House of the Dead, which transformed Dostoyevsky's vision of the world into an exciting collective drama.

From the early 1890s, Janáček led the mainstream of folklorist activity in Moravia and Silesia, using a repertoire of folk songs and dances in orchestral and piano arrangements. Most of his achievements in this field were published in 1899–1901 though his interest in folklore would be lifelong. His compositional work was still influenced by the declamatory, dramatic style of Smetana and Dvořák. He expressed very negative opinions on German neo-classicism and especially on Wagner in the Hudební listy journal, which he founded in 1884. The death of his second child, Vladimír, in 1890 was followed by an attempted opera, Beginning of the Romance (1891) and the cantata Amarus (1897).

Until 1895 he devoted himself mainly to folkloristic research and his early musical output was influenced by contemporaries such as Antonín Dvořák. His later, mature works incorporate his earlier studies of national folk music in a modern, highly original synthesis, first evident in the opera Jenůfa, which was premiered in 1904 in Brno. The success of Jenůfa (often called the "Moravian national opera") at Prague in 1916 gave Janáček access to the world's great opera stages. Janáček's later works are his most celebrated. They include operas such as Káťa Kabanová and The Cunning Little Vixen, the Sinfonietta, the Glagolitic Mass, the rhapsody Taras Bulba, two string quartets, and other chamber works. Along with Antonín Dvořák and Bedřich Smetana, he is considered one of the most important Czech composers.

The early orchestral works are influenced by Romantic style, and especially by orchestral works of Dvořák. In his later works, created after 1900, Janáček found his own, original expression.

Janáček composed his major piano works in a relatively short period of twelve years, from 1901 to 1912. His early Thema con variazioni (subtitled Zdenka's variations) is a student work composed in the styles of famous composers.

His married life, settled and calm in its early years, became increasingly tense and difficult following the death of his daughter, Olga, in 1903. Years of effort in obscurity took their toll, and almost ended his ambitions as a composer: "I was beaten down", he wrote later, "My own students gave me advice – how to compose, how to speak through the orchestra". Success in 1916 – when Karel Kovařovic finally decided to perform Jenůfa in Prague – brought its own problems. Janáček grudgingly resigned himself to the changes forced upon his work. Its success brought him into Prague's music scene and the attentions of Soprano Gabriela Horvátová, who guided him through Prague society. Janáček was enchanted by her. On his return to Brno, he appears not to have concealed his new passion from Zdenka, who responded by attempting suicide. Janáček was furious with Zdenka and tried to instigate a divorce, but lost interest in Horvátová. Zdenka, anxious to avoid the public scandal of formal divorce, persuaded him to settle for an "informal" divorce. From then on, until Janáček's death, they lived separate lives in the same household.

The operas of his mature period, Jenůfa (1904), Káťa Kabanová (1921), The Cunning Little Vixen (1924), The Makropulos Affair (1926) and From the House of the Dead (after a novel by Dostoyevsky and premièred posthumously in 1930) are considered his finest works. The Australian Conductor Sir Charles Mackerras became very closely associated with Janáček's operas.

In 1905, Janáček attended a demonstration in support of a Czech university in Brno, where the violent death of František Pavlík, a young joiner, at the hands of the police inspired his piano sonata, 1. X. 1905. The incident led him to further promote the anti-German and anti-Austrian ethos of the Russian Circle, which he had co-founded in 1897 and which would be officially banned by the Austrian police in 1915. In 1906, he approached the Czech poet Petr Bezruč, with whom he later collaborated, composing several choral works based on Bezruč's poetry. These included Kantor Halfar (1906), Maryčka Magdónova (1908), and Sedmdesát tisíc (1909).

Janáček's life in the first decade during the 20th century was complicated by personal and professional difficulties. He still yearned for artistic recognition from Prague. He destroyed some of his works, others remained unfinished. Nevertheless, he continued composing, and would create several remarkable choral, chamber, orchestral and operatic works, the most notable being the 1914 cantata, Věčné evangelium (The Eternal Gospel), Pohádka (Fairy tale) for cello and piano (1910), the 1912 piano cycle V mlhách (In the Mists) and his first symphonic poem Šumařovo dítě (A Fiddler's Child). His fifth opera, Výlet pana Broučka do měsíce, composed from 1908 to 1917, has been characterized as the most "purely Czech in subject and treatment" of all of Janáček's operas.

In 1916, he started a long professional and personal relationship with theatre critic, dramatist and translator Max Brod. In the same year, Jenůfa, revised by Kovařovic, was finally accepted by the National Theatre. Its performance in Prague in 1916 was a great success, and brought Janáček his first acclaim. He was 62. Following the Prague première, he began a relationship with singer Gabriela Horváthová, which led to his wife Zdenka's attempted suicide and their "informal" divorce. A year later (1917), he met Kamila Stösslová, a young married woman 38 years his junior, who was to inspire him for the remaining years of his life. He conducted an obsessive and (on his side at least) passionate correspondence with her, of nearly 730 letters. From 1917 to 1919, deeply inspired by Stösslová, he composed The Diary of One Who Disappeared. As he completed its final revision, he began his next 'Kamila' work, the opera Káťa Kabanová.

In 1917, he began his lifelong, inspirational and unrequited passion for Kamila Stösslová, who neither sought nor rejected his devotion. Janáček pleaded for first-name terms in their correspondence. In 1927, she finally agreed and signed herself "Tvá Kamila" (Your Kamila) in a letter, which Zdenka found. This revelation provoked a furious quarrel between Zdenka and Janáček, though their living arrangements did not change – Janáček seems to have persuaded her to stay. In 1928, the year of his death, Janáček confessed his intention to publicize his feelings for Stösslová. Max Brod had to dissuade him. Janáček's contemporaries and collaborators described him as mistrustful and reserved, but capable of obsessive passion for those he loved. His overwhelming passion for Stösslová was sincere but verged upon self-destruction. Their letters remain an important source for Janáček's artistic intentions and inspiration. His letters to his long-suffering wife are, by contrast, mundanely descriptive. Zdenka seems to have destroyed all hers to Janáček. Only a few postcards survive.

In 1920, Janáček retired from his post as Director of the Brno Conservatory but continued to teach until 1925. In 1921, he attended a lecture by the Indian philosopher-poet Rabindranath Tagore and used a Tagore poem as the basis for the chorus The Wandering MadMan (1922). At the same time, he encountered the microtonal works of Alois Hába. In the early 1920s, Janáček completed his opera The Cunning Little Vixen, which had been inspired by a serialized novella in the newspaper Lidové noviny.

In Janáček's 70th year (1924), his biography was published by Max Brod, and he was interviewed by Olin Downes for The New York Times. In 1925, he retired from teaching but continued composing and was awarded the first honorary doctorate to be given by Masaryk University in Brno. In the spring of 1926, he created his Sinfonietta, a monumental orchestral work, which rapidly gained wide critical acclaim. In the same year, he went to England at the invitation of Rosa Newmarch. A number of his works were performed in London, including his first string quartet, the wind sextet Youth, and his violin sonata. Shortly after, and still in 1926, he started to compose a setting to an Old Church Slavonic text. The result was the large-scale orchestral Glagolitic Mass. Janáček was an atheist, and critical of the organized Church, but religious themes appear frequently in his work. The Glagolitic Mass was partly inspired by the suggestion by a clerical friend and partly by Janáček's wish to celebrate the anniversary of Czechoslovak independence.

The world première of Janáček's lyrical Concertino for piano, two violins, viola, clarinet, French horn and bassoon took place in Brno on 16 February 1926. It was also performed at the Frankfurt Festival of Modern Music in 1927 by Ilona Štěpánová-Kurzová.

In 1927 – the year of the Sinfonietta's first performances in New York, Berlin and Brno – he began to compose his final operatic work, From the House of the Dead, the third Act of which was found on his desk after his death. In January 1928, he began his second string quartet, the Intimate Letters, his "manifesto on love". Meanwhile, the Sinfonietta was performed in London, Vienna and Dresden. In his later years, Janáček became an international Celebrity. He became a member of the Prussian Academy of Arts in Berlin in 1927, along with Arnold Schoenberg and Paul Hindemith. His operas and other works were finally performed at the world stages.

A comparable chamber work for an even more unusual set of instruments, the Capriccio for piano left hand, flute, two trumpets, three trombones and tenor tuba, was written for Pianist Otakar Hollmann, who lost the use of his right hand during World War I. After its première in Prague on 2 March 1928, the Capriccio gained considerable acclaim in the musical world.

The Janáček Philharmonic Orchestra was established in 1954. Today the 116-piece ensemble is associated with mostly contemporary music but also regularly performs works from the classical repertoire. The orchestra is resident at the House of Culture Vítkovice (Dům kultury Vítkovice) in Ostrava, Czech Republic. The orchestra tours extensively and has performed in Europe, the U.S., Australia, Japan, South Korea, and Taiwan. Its current music Director is Theodore Kuchar.

Janáček's friend and collaborator Václav Talich, former chief-conductor of the Czech Philharmonic, sometimes adjusted Janáček's scores, mainly for their instrumentation and dynamics; some critics sharply attacked him for doing so. Talich re-orchestrated Taras Bulba and the Suite from Cunning Little Vixen justifying the latter with the claim that "it was not possible to perform it in the Prague National Theatre unless it was entirely re-orchestrated". Talich's rearrangement rather emasculated the specific sounds and contrasts of Janáček's original, but was the standard version for many years. Charles Mackerras started to research Janáček's music in the 1960s, and gradually restored the composer's distinctive scoring. The critical edition of Janáček's scores is published by the Czech Editio Janáček.

Janáček's life has been featured in several films. In 1974 Eva Marie Kaňková made a short documentary Fotograf a muzika (The Photographer and the Music) about the Czech Photographer Josef Sudek and his relationship to Janáček's work. In 1983 the Brothers Quay produced a stop motion animated film, Leoš Janáček: Intimate Excursions, about Janáček's life and work, and in 1986 the Czech Director Jaromil Jireš made Lev s bílou hřívou (Lion with the White Mane), which showed the amorous inspiration behind Janáček's works. In Search of Janáček is a Czech documentary directed in 2004 by Petr Kaňka, made to celebrate the 150th anniversary of Janáček's birth. An animated cartoon version of The Cunning Little Vixen was made in 2003 by the BBC, with music performed by the Deutsches Symphonie-Orchester Berlin and conducted by Kent Nagano. A rearrangement of the opening of the Sinfonietta was used by the progressive rock band Emerson, Lake & Palmer for the song "Knife-Edge" on their 1970 debut album.

Janáček established a school of composition in Brno. Among his notable pupils were Jan Kunc, Václav Kaprál, Vilém Petrželka, Jaroslav Kvapil, Osvald Chlubna, Břetislav Bakala, and Pavel Haas. Most of his students neither imitated nor developed Janáček's style, which left him no direct stylistic descendants. According to Milan Kundera, Janáček developed a personal, modern style in relative isolation from contemporary modernist movements but was in close contact with developments in modern European music. His path towards the innovative "modernism" of his later years was long and solitary, and he achieved true individuation as a Composer around his 50th year.

Janáček always deeply admired Antonín Dvořák, to whom he dedicated some of his works. He rearranged part of Dvořák's Moravian Duets for mixed choir with original piano accompaniment. In the early years of the 20th century, Janáček became increasingly interested in the music of other European composers. His opera Destiny was a response to another significant and famous work in contemporary Bohemia – Louise, by the French Composer Gustave Charpentier. The influence of Giacomo Puccini is apparent particularly in Janáček's later works, for Example in his opera Káťa Kabanová. Although he carefully observed developments in European music, his operas remained firmly connected with Czech and Slavic themes.