Moréas was born in Athens, into a distinguished Greek family; he was the son of a judge. He received a French education, and came to Paris in 1875 to study law at the University of Paris. While in France, he began to move in literary circles, and became acquainted with Les Hydropathes, a group of French writers that included Alphonse Allais, Charles Cros, Guy de Maupassant, and Léon Bloy. He was also an acquaintance of the Greek artist Demetrios Galanis and the Romanian poet Ion Minulescu.
He published poetry in Lutèce and Le Chat noir, and collected his poems into two editions, Les Syrtes ("The Sandbanks") and Cantilènes, which were strongly influenced by Paul Verlaine.
He was initially an adherent of the school of Symbolism, and wrote the Symbolist Manifesto (1886), which he published in Le Figaro, in part as a means of distancing the aesthetic of the rising generation of young writers from the "decadent" label that the press had placed on them. He was considered one of the most important Symbolist poets until the early 1890s.
In 1891, as Symbolism became more openly associated with anarchism, he published Le Pèlerin passioné which rejected Northern European and Germanic influences, such as Romanticism (as well as some aspects of Symbolism), in favor of Roman and Ancient Greek influences. This work laid the foundation for the École Romane, whose aesthetic provided Charles Maurras with the ideological framework for the far-right Action Française.