Born in Palm, Pennsylvania, Eshbach grew up in Reading. He discovered science fiction at age 15 and began writing letters to the professional magazines, then started to write his own stories. The third story he wrote sold to Science Wonder Stories in 1929. While still writing his own stories and articles, he published two short-lived magazines in the early 1930s, Marvel Tales and The Galleon. He founded Fantasy Press, a small press which published the work of authors such as E. E. Smith, Jack Williamson, Robert A. Heinlein and John W. Campbell, Jr.. Fantasy Press published 32 books between 1946 and 1950, in limited print runs averaging 3,750 copies each. These constituted the first hardcover editions of many of these works, previously available only in used copies of magazines. Eshbach later formed Polaris Press in 1952 to publish 2 more books.
His memoirs, Over My Shoulder: Reflections on a Science Fiction Era, were published in 1982; and his last novel, The Scroll of Lucifer, in 1990. Other books included The Armlet of the Gods, The Land Beyond the Gate, The Sorceress of Scath and The Tyrant of Time. He also edited Of Worlds Beyond: The Science of Science Fiction Writing (Reading, PA: Fantasy, 1947), the first book-length work on science fiction writing from a professional point of view.