Disneyland opened on July 17, 1955, without a haunted mansion.
Within three years, Walt decided to expand Disneyland, to include a new land called "New Orleans Square", which would contain new shops, restaurants, and a haunted mansion. The foundation for the façade of the Plantation style haunted mansion took place in 1962.
Early concepts for a neglected looking house were replaced with Walt's preference, a clean, well-preserved façade, which matched the pristine look of the rest of the park. Walt said "We'll take care of the outside and let the ghosts take care of the inside".
Even though the façade was completed, it was an empty shell for years due to storyline problems and other distractions such as the 1964 New York World's Fair, where Disney built the first version of "It's a Small World", among other state of the art attractions.
On December 15, 1966, Walt Disney died of lung cancer.
With the loss of their great visionary, the Imagineers debated over design concepts for the haunted mansion, such as whether the haunted mansion experience should be scary, or funny. In the end, imaginers Marc Davis (funny), and Claude Coats (scary), combined their ideas, so the ride begins with spooky sets and chilling concepts, and eventually becomes "a swinging wake", full of silly spooks, and ghostly gags.
Finally, after years of planning the New Orleans Square Haunted Mansion opened its doors, on August 9, 1969.
Thank you, Uncle Walt, for creating the haunted mansion(s), which inspired me (and many others) to pursue a career in theme park entertainment design!