Mechanus was the jungle homeworld of the Mechonoids.
Shortly after encountering the Mechonoids, the Daleks set the rogue planet Skardal on a collision course with their home world. The Zerovians' robot agent 2K prevented Skardal from destroying Mechanus. (COMIC: The Rogue Planet, Impasse)
Humans sent Mechonoids to Mechanus to prepare the planet for colonisation, but Earth got caught up in an interplanetary war and the colonists never arrived. Steven Taylor crashlanded on the planet and was taken prisoner by the Mechonoids. The Mechonoids and their city were destroyed by the Daleks, and Steven departed in the Doctor's TARDIS. (TV: The Chase)
Behind the scenes
According to non-narrative material in DWBIT 3, an account was taken of the Daleks' battle between the Mechonoids during the former's mission to capture the Doctor.