The Martians, living secretly on Earth after evacuating their dying home world, have been trying to guide the development of human civilization for thousands of years.

[4] Boucher and McComas praised the novel, saying "The warmth, depth and perception of a true novelist have given wholly new life to the shopworn [premise]; and the fallible humanity of Pangborn's Martian Observers (from whose viewpoint the story is told) removes this from the paranoid Superman category and makes it a distinguished and moving novel of people [and] their common problems of free will".

[5] Groff Conklin declared A Mirror for Observers a "beautiful and moving book, saying that "Despite the bigness of the theme, the story is told in little details which make the tragedy all the more impressive."

[7] New Worlds reviewer Leslie Flood complimented the British edition as "a book which has given me as much pleasure to read as any other in the past", citing "the tender commiseration for humanity and the understanding objectivity of Elmis himself".

[8] Jo Walton termed Mirror "deservedly a classic", saying "It’s the mood that I remember and that brings me back to it, the Martians and the humans, the tensions, the sense of time".