Photius describes him as a threptos, a kind of assistant of servile origin, to Cinnaeus and states that he was later a secretary to Heraclides Lembus.
Cinnaeus served as a counselor to Ptolemy VI; Heraclides is best known for negotiating the treaty that ended Antiochus IV's invasion of Egypt in 169 BC.
At the conclusion of his On the Erythraean Sea, he apologizes for being unable to complete his work "since our age is unable to similarly bear the toil" and "as a result of the disturbances in Egypt" he could no longer access the official records (a fragment cited by Photius in his Bibliotheca Cod.
Extracts from the first book of his Erythraean Sea, written in the first person and advocating a military campaign into the lands south of Egypt, led early scholars to deduce that Agatharchides was an important political figure of his time, and served as a guardian to one of the sons of Ptolemy VIII.
Dodwell endeavored to show that it the younger son, Alexander, and objects to Soter, that he reigned conjointly with his mother.
Wesseling and Henry Fynes Clinton think the elder brother to be the one meant, for Soter was more likely to have been a minor on his accession in 117 BC than Alexander in 107 BC, ten years after their father's death; the second edition of the Oxford Classical Dictionary article on Agatharchides agrees that the son was Soter.
However at least as early as 1810, when B. G. Niebuhr pointed out that these excerpts were from a speech, and not part of the narrative of his book, this theory has been recognized as conflicting with other known historical facts.
Material from this book is quoted directly or indirectly by Diodorus Siculus, Strabo, Pliny the Elder, Aelian (Claudius Aelianus), Josephus[1] and other authors.
In the composition of his speeches Agatharchides was an imitator of Thucydides, whom he equalled in dignity and excelled in clearness.