Era amigo do lexislador Nicodoro de Mantinea. Cando esta vila da Arcadia se puxo baixo a protección de Esparta, marchou a Atenas, onde pasou de ser poeta compositor de ditirambos e himnos relixiosos a ser un escéptico e ateo. Segundo Sexto Empírico volveuse ateo ao comprobar como un inimigo seu partía dun xuízo sen castigo algún logo de cometer perxurio ao xurar sobre os deuses ser inocente, o que lle bastou para saír sen pena. Dicía "Se a inmoralidade pode permanecer impune, para que crer en deuses que velan a virtude humana?".
Foi discípulo de Demócrito (segundo algunhas fontes foi comprado por este como escravo e converteuse no seu estudante) e contou os secretos misterios de Eleusis a todo o mundo e tratou de disuadir á xente para que non se iniciase, o que lle custou unha condena á morte, polo que marchou ao exilio no Peloponeso no 415; segundo Diodoro, púxose prezo á súa cabeza, pois Aristófanes escribiu que se prometía un talento pola súa morte e dous se era traído vivo.
O escritor romano Cicerón, no século I a.C., conta como un amigo de Diagoras intentou convencelo da existencia dos deuses, falándolle da cantidade de pinturas votivas existían de persoas salvadas de naufraxios facendo promesas aos deuses, ao que Diágoras replicou que "en ningunha parte hai pinturas dos que naufragaron e afogaron." Cicerón conta outra historia, Diágoras viaxaba nun navío que se encontraba nunha forte tormenta, a tripulación pensaba que sufrian ese mal tempo por levar a un ateo a bordo. El entón preguntoulles se o resto dos navíos que sufrían a tormenta tamén transportaban un Diágoras.
↑The poet Diagoras of Melos was perhaps the most famous atheist of the fifth century. Although he did not write about atheism, anecdotes about his unbelief suggest he was self-confident, almost teasing, and very public. He revealed the secret rituals of the Eleusinian mystery religion to everyone and "thus made them ordinary," that is, he purposefully demystified a cherished secret rite, apparently to provoke his contemporaries into thought. In another famous story, a friend pointed out an expensive display of votive gifts and said, "You think the gods have no care for man? Why, you can see from all these votive pictures here how many people have escaped the fury of storms at sea by praying to the gods who have brought them safe to harbor." To which Diagoras replied, "Yes, indeed, but where are the pictures of all those who suffered shipwreck and perished in the waves?" A good question. Diagoras was indicted for profaning the mysteries, but escaped. A search was out for him throughout the Athenian empire, which indicated that the charges were serious, but he was not found. Jennifer Michael Hecht Doubt: A HistoryHarper San Francisco 2003 chapter: Whatever Happened to Zeus and Hera?, 600 BCE-1 CE páx. 9-10. ISBN 0-06-009795-7
↑As regards, first of all, the allegation that we are atheists-for I will meet the charges one by one, that we may not be ridiculed for having no answer to give to those who make them-with reason did the Athenians adjudge Diagoras guilty of atheism, in that he not only divulged the Orphic doctrine, and published the mysteries of Eleusis and of the Cabiri, and chopped up the wooden statue of Hercules to boil his turnips, but openly declared that there was no God at all. But to us, who distinguish God from matter, and teach that matter is one thing and God another, and that they are separated by a wide interval (for that the Deity is uncreated and eternal, to be beheld by the understanding and reason alone, while matter is created and perishable), is it not absurd to apply the name of atheism? If our sentiments were like those of Diagoras, while we have such incentives to piety-in the established order, the universal harmony, the magnitude, the colour, the form, the arrangement of the world-with reason might our reputation for impiety, as well as the cause of our being thus harassed, be charged on ourselves. Athenagoras the Athenian, A Plea for the Christians, Chapter 4
↑It was about that time ["415 B.C."] that the poet Diagoras of Melos ["active in Athens in the last decades of the 5th cent. BC" (Ox. Classical Dict., 1996)] was proscribed for atheism, he having declared that the non-punishment of a certain act of iniquity proved that there were no Gods. It has been surmised, with some reason, that the iniquity in question was the slaughter of the Melians by the Athenians in 416 B.C., and the Athenian resentment in that case was personal and political rather than religious. For some time after 415 the Athenian courts made strenuous efforts to punish every discoverable case of impiety; and parodies of the Eleusinian mysteries were alleged against Alkibiades and others. Diagoras, who was further charged with divulging the Eleusinian and other mysteries, and with making firewood of an image of Herakles, telling the god thus to perform his thirteenth labour by cooking turnips, became thenceforth one of the proverbial atheists of the ancient world, and a reward of a silver talent was offered for killing him, and of two talents for his capture alive; despite which he seems to have escaped. A History of Freethought, Ancient and Modern, to the Period of the French Revolution, J.M. Robertson, Fourth Edition, Revised and Expanded, In Two Volumes, Vol. I, Watts, 1936. p173 - 174