When was it discovered Mars had its own Mountain?

Answer 1

Since Mars was discovered to be a world rather than just a point of light in the sky, it's been assumed that it had mountains.

If you're asking when Olympus Mons was discovered, it was first directly observed to be a mountain in 1971 by the Mariner 9 spacecraft. Prior to that it had been known as Nix Olympica (Olympian Snow) due to its coloration, and had been suspected to be a mountain as early as the 19th century since it was one of the few features that remained visible during dust storms, implying that it was tall enough for its top to be visible even when the lowlands were obscured.