Hidden Mickeys: DisneyLAND or Disney WORLD?

I don't know where it started, but they are in both parks. Disney World has a book you can buy that shows where some are (certainly not all, because I found some not in the book). I don't know if Disneyland has a book as well. The book was not written by anyone at Disney, but a private entity who then got permission to sell the book on Disney property. That's why I'm not sure if there's a book out for Disneyland.

Well, since DisneyLAND was built in 1955, the practice must have started there. Disney World's Magic Kingdom didn't open until 1971.

According to what I've found, there are Hidden Mickeys at both parks as well as in some Disney movies. I don't know what park it began in, but I think that it is likely that they were put in both parks around the same time. All I know is that the first know Hidden Mickey was made by Arlen Miller, who wrote an article on Hidden Mickeys for WDW's Eyes and Ears (a Cast Member weekly publication) in 1989. The article listed Hidden Mickeys found in the Disney theme parks. Months later the author was contacted by Disney News for more information, and the resulting article made the news of Hidden Mickeys spread worldwide.