Earth's twin planet. It was the Cybermen's homeworld. It had orbited around the Sun millions of years in the past. The Mondasian civilization developed more quickly than Earth's and Mondasians visited our planet in prehistoric times. Later, for reasons yet unknown, Mondas left the Solar System. It was during that period of wandering that the Mondasians evolved into creatures of ruthless logic; their organs were replaced by cybernetic parts until they became Cybermen. At a certain point during their space journey, some Cybermen left Mondas to form a planetary empire, enslaving other civilisations and turning them into Cybermen.
The Mondasian Cybermen eventually returned to the Solar System in 1986, planning to drain energy from Earth and turn its population into Cybermen. When they saw that Mondas was absorbing too much energy and thus risked destruction, the Cybermen then decided to destroy Earth with the Z-Bomb. However, their plan was thwarted by the First Doctor, and Mondas was destroyed, along with its inhabitants (DD).
Galactic Cybermen from the future tried to prevent Mondas' destruction by attempting to launch Halley's Comet into the Earth before the above events took place, but they were defeated by the Sixth Doctor and the Cryons (6T).