The phrase man from Mars denotes a hypothetical observer of human behaviour and society whose perspective would be entirely detached and objective.

The earliest occurrence of this phrase that I have found is from the following article, published in The Morning Call (San Francisco, California, USA) of Saturday 19^th March 1892:

Putting on the Screws.
Driving California Merchandise to Asia via Northern Ports.

San Francisco freight is going 800 miles out of its way in order to be reshipped and transported in foreign steamers. Merchandise from San Francisco bound for China and Japan is first carried in ships 800 miles north to Vancouver, there to be taken to its destination in vessels flying the British flag.
Why?
Because it is cheaper to ship in this roundabout way than by the heavily subsidized American line of steamers. San Francisco merchants are patriotic—on Fourth of July and all other parade days—but they conduct business on business principles, and find it much cheaper to ship or receive oriental freight by British steamers.
One unacquainted with the situation would be inclined to doubt this statement, and the Man from Mars would be inclined to ridicule a commercial system that would make it cheaper to ship by an indirect than a direct route. But the facts and figures to prove the assertion are abundant, and there are hundreds of San Francisco merchants who know from bitter experience the truthfulness of these strange things.

But man from Mars had been used a few years earlier, in a text which simply indicated that the idea of a Martian’s visit to the Earth had already been overused; this text is the review of the May 1887 issue of The Century: Illustrated Monthly Magazine (New York: The Century Co.)—review published in the Boston Daily Advertiser (Boston, Massachusetts, USA) of Saturday 30^th April 1887:

If the much hackneyed and essentially tiresome man from Mars should appear upon this planet unexpectedly, the best way to introduce him to our civilization would be to give him a copy of the May Century to read.

And, indeed, one Ernest Menkler used the idea of a visitor from Mars or Jupiter in a letter published in The Daily Inter Ocean (Chicago, Illinois, USA) of Sunday 16^th November 1890:

MATTERS MONUMENTAL.

Chicago, Nov. 12.—To the Editor.—After all there is only one place in the world where you can always find good people, and that is in the cemetery. Everybody takes his right place there, and no nonsense about it. There is no struggling for power, no ridiculous striving for fame. The people underground are much better behaved than we, and yet I suppose there is no place in the world where there is more lying done. If you do not believe it, read the tombstones. Fancy a celestial visitant, a man from Mars or Jupiter, come down on a trip to earth, and suppose that he lit in one of our well-regulated grave yards and began to read the various inscriptions on the tombstones; and suppose before he went any further he winged his way back through the realms of space to the spot whence he came, what a glowing account would he have to give of the people of earth. There is no doubt he would be for bringing back the whole planet with him. They would be sure to settle here. There is no doubt of that if only for the privilege of associating themselves with such fine underground society.

In 1891 and 1892, three fictitious prose narratives, whose respective titles contained man from Mars, told of visits either from, or to, Mars. They had for common theme that we are far behind Mars in discoveries in the material and spiritual worlds.

These three narratives are:

1-: Man from Mars. Aubrey Stayne’s Revenge, a short story written for The Chicago Herald (Chicago, Illinois, USA) by the U.S. novelist and poet Edgar Fawcett (1847-1904), and published in The Chicago Herald on Saturday 30^th May 1891. In this short story, Aubrey Stayne befriends a man from Mars, whom he has rescued from the wreckage of his spaceship, an “aerial car”. This Martian, remarkable for his “facial beauty and physical grandeur”, becomes fluent in English after a few days, thanks to “an intelligence of no earthly grasp”, which makes Stayne recoil “before mental faculties whose reach and penetration he had thus far imagined only”. The following is a passage from this short story:

Chalco (by which name Stayne had got to know him) declared himself to be an inhabitant of the planet Mars. For many years, he stated, that world had been desirous of holding intercourse with our own. Enormous lengths of structure had been reared from a metal of peculiar lightness and suitability for purposes of building. From this same metal had been constructed the car in which he had shot, by electric potencies not explainable here, toward our comparatively adjacent orb. Electricity in Mars had reached great heights of conception. This Chalco, the younger son of a powerful ruler, had been deemed almost despicable for his want of mental power. More than that, his high rank had made his love for a certain lady held below him in the social grade to be considered an almost treasonable offence. Chalco was in his way a pariah, and when the tremendous problem of possible interplanetary communion had approached solution his offer to mount the perilous car and make the audacious voyage had been received with sombre assent.

2-: A Man from Mars, by William McDonnell (1814-1900), a novel which does not seem to have ever been published, but whose issuing was announced in The St. Louis Republic (St. Louis, Missouri, USA) of Saturday 5^th December 1891:

A theosophical novel by Mr. W. McDonnell, author of the very successful “Exeter Hall,” and of “Heathen of the Heath,” is announced for early issue by John A. Taylor & Co. of New York. The title selected for the forthcoming book is “A Man from Mars,” and the story is said to run on the lines of Edward Bellamy’s sociological “Looking Backward.” * The work purports to describe a visit to the planet Mars by two adepts in theosophy by occult powers. They find a perfect social system in operation amongst the inhabitants of Mars—society being organized on the same principles as those laid down in Mr. Bellamy’s story.

* Looking Backward 2000–1887 (1888) is a utopian novel by the U.S. author and journalist Edward Bellamy (1850-1898). It tells the story of Julian West, who falls into a hypnosis-induced sleep and wakes up 113 years later in the same location, Boston, Massachusetts: it is the year 2000, and the United States has been transformed into a socialist utopia.

3-: The Man from Mars. His Morals, Politics and Religion (San Francisco: Bacon & Company, Printers. 1891), by Thomas Blot, pseudonym of the U.S. author William Simpson (1828-1910); the following review of this novel was published in The Morning Call (San Francisco, California, USA) of Sunday 21^st February 1892:

This book is one that is devoted to the discussion of social questions, including religion, labor and land-ownership. The ideas set forth in this book were no doubt suggested by Bellamy. The author, who represents himself as a recluse settled down in the mountains where he can commune with nature and understand its greatness, tells of a celebrated visitor who comes to him and engages him in conversation, telling him of all that is going on on the planet Mars, whence he made a trip to earth. Through this visitor, who draws comparisons, the author calls attention to what he considers many deficiencies in social, religious and labor problems on this earth. He shows out of the mouth of this celestial visitor, “The Man from Mars,” that the people of this mundane sphere are at least half a century behind those who inhabit the planet whence he came. By him he shows that if the people of this earth were only as far advanced as those in the planet above us, we should be living in a perfect paradise. The object of the writer is to show that if the principles that govern the inhabitants of Mars, as represented by his celestial visitor, were those that governed us, many of the problems that have worried people for many years would be solved, and that there would no longer be any strife in religion or social matters, and that the greatest of all problems, the labor one, would be understood by all, and the conflict that now exists between labor and capital would be at an end, for all would be arranged on equitable grounds. The whole matter is presented from a novel point of view and the book is worth the reading for the ideas that are suggested.