Uatu first appeared (Fantastic Four #13, 1963); the Watchers as a species appeared (Strange Tales #53, 1964)
Their home-world exist in an unknown galaxy, far beyond the Milky Way Galaxy. It was covered with lush vegetation and bears no artifacts of habitation.
The Watchers possessed advanced technology existing billions of years before most other species. They used their advanced technology to create temporary dwellings which they disassemble after use.
Ruled under leader The One, who is physically larger than the other Watchers.
The Watchers were giant humanoid beings, that dress in the clothing of ancient monks or priest.