* The Haunted Mansion at Disneyland: This was the first Haunted Mansion ride to open, in 1969.
* The concept of a Haunted Mansion ride: This was first conceived in the 1950s, before the Disneyland version was even built.
So, if you mean the first physical Haunted Mansion ride to be constructed and open, then Disneyland's Haunted Mansion was the first. But if you mean the first time the idea of a Haunted Mansion ride was thought of, then that happened earlier.
It's important to note that the original concept for the Haunted Mansion was much different than the ride we know today. It was originally envisioned as a more traditional dark ride with a story about a ghost who is trying to find his lost love. However, the Imagineers eventually decided to go with a more comedic and spooky approach, which resulted in the ride we know and love today.