In 1870, Fauré enlisted in the army and took part in the action to raise the Siege of Paris during the Franco-Prussian War. During the Paris Commune he stayed at Rambouillet and in Switzerland, where he taught at the transported École Niedermeyer. When he returned to Paris in October 1871, he was appointed assistant organist at Saint-Sulpice as accompanist to the choir, and became a regular at Saint-Saëns' salon. Here he met many prominent Parisian musicians and with those he met there and at the salon of Pauline Viardot he formed the Société Nationale de Musique. In 1874, Fauré stopped working at Saint-Sulpice and began to fill in at the Église de la Madeleine for Saint-Saëns during his many absences. When Saint-Saëns retired in 1877, Fauré became choirmaster. In the same year he became engaged to Marianne Viardot, daughter of Pauline Viardot, but the engagement was later broken off by Marianne. Following this disappointment he travelled to Weimar, where he met Liszt, and Cologne in order to see productions of Richard Wagner's Der Ring des Nibelungen. Fauré admired Wagner, but was one of few composers of his generation not to come under his influence.

In 1883, Fauré married Marie Fremiet, with whom he had two sons. In order to support his family Fauré spent most of his time in organising daily services at the Église de la Madeleine and teaching piano and harmony lessons. He only had time to compose during the summers. He earned almost no money from his compositions because his publisher bought them, copyright and all, for 50 francs each. During this period Fauré wrote several large scale works, in addition to many piano pieces and songs, but he destroyed many of them after a few performances, only retaining a few movements in order to re-use any motifs.

During his youth Fauré appeared to be very cheerful, but his broken engagement combined with his perceived lack of musical success precipitated bouts of depression which he described as "spleen". In the 1890s, however, his fortunes improved somewhat. He had a successful trip to Venice where he met with friends and wrote several works. In 1892, he became the inspector of the music conservatories in the French provinces, which meant he no longer had to teach amateur students. In 1896, he finally became chief organist at the Église de la Madeleine, and also succeeded Jules Massenet as composition instructor at the Conservatoire de Paris. At this post he taught many important French composers, including Maurice Ravel and Nadia Boulanger.

From 1903 to 1921, Fauré was a critic for Le Figaro. In 1905, he succeeded Théodore Dubois as director of the Paris Conservatory. He made many changes at the Conservatoire, leading to the resignation of a number of faculty members. This position meant that he was better off in terms of income, and he also became much more widely known as a composer.

Fauré was elected to the Institut de France in 1909, but at the same time he broke with the Société Nationale de Musique, and supported the rogue group which formed out of those ejected from the Société, mainly his own students. During this time Fauré developed ear trouble and gradually lost his hearing. Sound not only became fainter, but it was also distorted, so that pitches on the low and high ends of his hearing sounded like other pitches. He made efforts to conceal his difficulty, but was eventually forced to abandon his teaching position.

His responsibilities at the Conservatoire, combined with his hearing loss, meant that Fauré's output was greatly reduced during this period. During World War I Fauré remained in France. In 1920, at the age of 75, he retired from the Conservatoire mainly due to his increasing deafness. In this year he also received the Grand-Croix of the Légion d'honneur, an honor rare for a musician. He suffered from poor health, partially brought on by heavy smoking. Despite this, he remained available to young composers, including members of Les six, who were devoted to him.

Gabriel Fauré died in Paris from pneumonia in 1924. He was given a state funeral at the Église de la Madeleine and is buried in the Passy Cemetery in Paris.

Gabriel Fauré is regarded as the master of the French art song, or mélodie. His works ranged from an early romantic style, when in his early years he emulated the style of Mendelssohn and others, to late 19th century Romantic, and finally to a 20th century aesthetic. His work was based on a strong understanding of harmonic structures which he received at the École Niedermeyer from his harmony teacher Gustave Lefèvre, who wrote the book Traité d'harmonie (Paris, 1889), in which Lefèvre sets forth a harmonic theory which differs significantly from the classical theory of Jean-Philippe Rameau in that seventh and ninth chords are no longer considered dissonant, and the mediant can be altered without changing the mode. In addition, Fauré's understanding of the church modes can be seen in various modal passages in his works, especially in his melodies.

In contrast with his harmonic and melodic style, which pushed the bounds for his time, Fauré's rhythmic motives tended to be subtle and repetitive, with little to break the flow of the line, although he did utilize subtle large scale syncopations, similar to those found in Brahms's works. Aaron Copland referred to him as the 'French Brahms'. It has also been posited by Jerry Dubins in Fanfare Magazine that Gabriel Fauré is the "missing link" between Brahms and Debussy.

Fauré's piano works often use arpeggiated figures with the melody interspersed between the two hands, and include finger substitutions natural for organists. These aspects make them daunting for some pianists, but they are nonetheless central works.

Fauré was a prolific composer, and among the most noteworthy of his works are his Requiem, the opera Pénélope, the orchestral suite Masques et bergamasques (based on music for a dramatic entertainment, or divertissement comique), and music for Pelléas et Mélisande. He also wrote chamber music; his two piano quartets are particularly well known. Other chamber music includes two piano quintets, two cello sonatas, two violin sonatas, a string quartet, a piano trio and a number of piano pieces including the Nocturnes. He is also known for his songs, such as Après un rêve, Les roses d'Ispahan, En prière, and several song cycles, including La bonne chanson with settings of poems by Verlaine, and L'horizon chimérique.

The Requiem, Op. 48, was not composed to the memory of a specific person but, in Fauré's words, "for the pleasure of it." It was first performed in 1888. Fauré is thought not to have had strong religious beliefs. It has been described as "a lullaby of death". In setting his requiem, he left out the Dies irae, though the reference to the day of judgment appears in the Libera me, which, like Giuseppe Verdi, he added to the normal requiem mass. Several slightly different versions of the Requiem exist, and these have given rise to a number of different recordings. Personal grief may have influenced the composition as it was started after the death of his father, and before it was completed, his mother died as well. The Requiem can thus be seen as an expression of Fauré's personal tragedy written after the death of his parents. The Requiem is also acknowledged as a source of inspiration for the similar setting by Maurice Duruflé.

Fauré wrote his Dolly Suite for piano four-hands between 1894 and 1897 and dedicated it to Hélène, daughter of the singer Emma Bardac, with whom he was having an affair; he also wrote La bonne chanson for Emma herself. In the UK, the first piece, Berceuse, from the Dolly Suite became known to several generations of children as the closing music for the BBC Home Service radio programme Listen with Mother, which was broadcast from 1950 to 1982. His music is used in "Act I: Emeralds" of George Balanchine's ballet Jewels (1967).