During my lifetime, the idea of a Martian population has been considered so unlikely as to be ruled out.
However, looking back at fictional works, such as early Twilight Zone episodes, or Ray Bradbury's "The Martian Chronicles", the idea seems to be treated as entirely plausible. At least, possible enough to support speculative fiction without relying on some "alternate universe" device.
At what point did the popular understanding of Mars rule out this sort of speculation? Were there particular exploration missions that led to a large shift in public consciousness?
William Wallace Campbell put forth an extensive, decades-spanning effort to prove against the weight of accepted turn-of-the-20th-century science to demonstrate that the Martian atmosphere had negligible water and oxygen. His 1894 and 1910 demonstrations still didn't quell public certainty, of course. In 1925, Charles St. John and Walter Adams confirmed Campbell's findings.
But since those scientists were looking at present-day Mars, there was still the possibility of ancient civilizations. And indeed, the 19th century fascination with what appeared to be (from observation) a series of constructed canals on Mars was the biggest driver of the public imagination for intelligent life. This was a big influence on H.G. Wells in the foundations of War of the Worlds.
Scientists in the first decade of the 20th century showed that the apparent presence of canals on Mars could very well be an optical illusion. Observatory work in France and Italy in 1909 confirmed this: no constructed canals, just geological formations.
Mariner 4's 1965 Martian observation mission gave us up-close photographs of a barren landscape formed by meteorites and geology, and confirmation of the lack of oxygen and water and protective magnetic field.
Speculative fiction is a wonderful genre for making the impossible and maybe-possible seem not only plausible but mundane. :)