His father was Francis Magnard, a journalist who became editor of Le Figaro, France's top newspaper, from 1879 to his death in 1894. Magnard's mother died when he was young. He had an easy, upper middle-class background in which musical training for social (not professional) attainment was standard. Eventually, he turned against the ease of his station, not hesitating to show his contempt for his bourgeois class, and seemingly choosing the rougher path whenever the opportunities arose.
After finishing secondary school he lived a monastic life for a while at Ramsgate Abbey in England, did a tour of military service as an officer, and then entered law school back in France. Although he had not demonstrated proficiency at an instrument or as a composer, he decided to make music a career, turning down offers of jobs in journalism from his father or from other journalists who obviously wanted to curry favor with the powerful elder Magnard.
Like many young Frenchmen of the time, Magnard made the pilgrimage to Wagner's theater and shrine at Bayreuth, Bavaria, where he was duly bowled over by Tristan und Isolde. Incidentally, the name he preferred is only coincidentally Wagnerian; it was his godfather's name. Magnard entered the Conservatoire and studied with Dubois and Massenet. He didn't like either teacher, but he did graduate with first prize in harmony. He met César Franck and studied for four years with Vincent d'Indy. He was highly influenced by their music, which showed a tendency to mix Wagnerian harmony with Classical forms and to rely on cyclic themes (i.e., main musical ideas that recur from one movement to later ones) in their construction. Magnard adopted this sort of form, but initially wrote thickly over-orchestrated music, exemplified by his First Symphony.
He also wrote a one-act opera, Yolanda. It was premiered in 1892 in Brussels, while the First Symphony was first heard in Angers in 1894. He had to endure not-so-quietly whispered innuendos that these works could not make it on their own merits, but were produced solely to curry favor with his powerful journalist father. He did not endure these rumors gracefully, and he developed even more of a tendency to humorlessness and lack of tact and social grace.
In 1894 his father died. Seemingly in reaction to this blow, he wrote his Chant funèbre, a work of powerful but severely restrained emotions that seemed to clarify his artistic direction. In 1896, he became a counterpoint teacher at the Schola Cantorum. In the same year he married Julia Creton, and set to work on his Second Symphony. At the time of his marriage, he wrote for her to play a piano suite called Promenades, which is a rare light-hearted piece from his pen, happily recalling places in Paris they enjoyed together.
Magnard alienated a segment of the public by his reaction to the Dreyfus affair-polarized France. Magnard, disgusted with the Army's blatant anti-Semitism, publicly resigned his reserve commission.
In 1900, he wrote an opera, Guercoeur, a post-Wagnerian work set in legendary times. During his life it received only two separate concert performances of one individual act each. It was not produced on stage or heard in its entirety until April 1931, by which time some parts of it had gone missing and had to be reconstructed by Magnard's friend, the composer Guy Ropartz.
Failure to get Guercoeur produced further embittered Magnard. In addition, he began to go deaf. Much as in the case of Beethoven, this made his exterior even more forbidding. It prompted him to retreat to the quiet and security of his country estate at Baron. In a major career miscalculation, he fired his publisher because he thought it was unseemly to advertise and publicize himself. He self-published his music thereafter. Without an agent and publisher to promote and network his music, he lost opportunities for performance and began to be forgotten except by his strongest advocates. Still, his chamber music received some performance. Another opera, Bérénice, was performed at the Opéra-Comique in 1911, but again fell flat with an unprepared public.
In August 1914, when World War I began, Magnard sent his wife and children west for safety, but he remained at Baron for a while. On September 3, he was working in his study, alone in the house, when he spotted a troop of German cavalry riding on his estate grounds. He seized his old Army rifle and shot two of them dead from his upstairs window. The Germans returned fire and burned down the house, where his body was later found in the ashes. Also lost in the fire were all existing materials for the opera Yolanda, the full score of two acts of Guercoeur and a new set of 12 songs.