Information on his biography is limited. He was born in Cyme, Aeolia and, together with the historian Theopompus, was a pupil of Isocrates, in rhetoric. He does not seem to have made much progress as a speaker and at the suggestion of Isocrates himself that he took up literary composition and the study of history. According to Plutarch, Ephorus declined Alexander the Great's offer to join him on his Persian campaign as the official historiographer. His son Demophilus followed in his footsteps as a historian.
The fruit of his labours was a set of 29 books, his universal history. The whole work, edited by his son Demophilus—who added a 30th book—contained a summary description of the Sacred War, along with other narratives from the days of the Heraclids up until the taking of Perinthus in 340 BC by Philip of Macedon, covering a time span of more than seven hundred years. According to Polybius, Ephorus was the first historian ever to author a universal history. For each of the 29 separate books Ephorus wrote a prooimion. The work was probably simply named Historiai, and followed a thematic, rather than a strictly chronological order in its narrative. Diodorus Siculus was largely responsible for preserving this work for posterity, by copying large parts of his writings. Book 30, covering the years 356–340 BC, was added by Demophilus quite probably after his death. The excerpts of their writings in Diodorus constitute the only continuous narrative on the history of Greece between 480 and 340 BC.
It is clear that Ephorus made critical use of the best authorities, and his work, highly praised and much read in Antiquity, was freely drawn upon by Diodorus Siculus and other compilers. Strabo attaches much importance to his geographical investigations, and praises him for being the first to separate the historical from the simply geographical element. In his Geographica, Strabo quotes Ephorus at length. Polybius, while crediting him with a knowledge of the conditions of naval warfare, ridicules his description of the battle of Mantinea as showing ignorance of the nature of land operations.
Besides the universal history, Ephorus wrote an Epichorios logos (Ἐπιχώριος λόγος), a patriotic essay in which he praised the traditions of Cyme. He also wrote Peri heurematon (Περὶ εὑρημάτων), a book about inventions, and Peri lexeos (Περὶ λέξεως), "On Style".
Other works attributed to him were:
Despite having written all these works, nothing but isolated fragments survived from the ancient world. His entire work has been lost.
According to the Encyclopædia Britannica Eleventh Edition, his surviving writings all show a certain lack of passion, in spite of his keen interest in matters of style, and of political partisanship, except for his enthusiasm for Cyme. According to ancient writers, he was respected as an able and thorough, though somewhat dull historiographer. He was commended for drawing (though not always) a sharp line of demarcation between the mythical and historical; he even recognized that a profusion of detail, though lending corroborative force to accounts of recent events, is ground for suspicion, in reports of far-distant history. His style was high-flown and artificial, as was natural considering his early training, and he frequently sacrificed truth to rhetoric effect. However, according to Dionysius of Halicarnassus, he and Theopompus were the only historical writers whose language was accurate and finished.