Hermarchus was a son of Agemarchus, a poor man of Mytilene (in insular Greece), and was at first brought up as a rhetorician, but afterwards became a faithful disciple of Epicurus, who left to him his garden, and appointed him his successor as the head of his school, about 270 BC. He died in the house of Lysias at an advanced age, and left behind him the reputation of a great philosopher. Cicero has preserved a letter of Epicurus addressed to him.
Diogenes mentioned from a letter written by Epicurus, "All my books to be given to Hermarchus. And if anything should happen to Hermarchus before the children of Metrodorus grow up, Amynomachus and Timocrates shall give from the funds bequeathed by me, so far as possible, enough for their several needs, as long as they are well ordered. And let them provide for the rest according to my arrangements; that everything may be carried out, so far as it lies in their power."