McDonald's / Stanley Clark Meston

The oldest operating McDonald's is located in Downey, CA. It was the 3rd McDonald’s built, and opened in 1953. It was the second restaurant franchised by brothers Richard and Maurice McDonald. The first McDonald's was in San Bernardino. 

In the early 1950s, Richard and Maurice McDonald hired commercial architect Stanley Clark Meston to design a drive-in (as opposed to drive thru) hamburger stand. Meston and his draftsman, Charles Fish, came up with the prototype franchise design in 1953. 

Fish ended up moving to San Diego and teaming up with James Bird and Walter Fujimoto. See their profile at Modern San Diego

Source: (Charles Fish collection) via Alan Hess - Which also includes a more in depth post about the Downey McDonalds.

Roger Williams and Bud Landon purchased franchise rights for the Downey stand from the McDonald brothers in 1953, prior to Ray Kroc's buying the chain in 1961. Therefore, they were not required to adhere to standard McDonald's franchise rules, including modernization standards. Along with keeping the original design, the Downey McDonald's refused to add Big Macs to the menu for years after they were introduced in 1968.

In 1984, Pep Boys bought the site, which included the restaurant. They have refused to sign off on adding the building to the National Register of Historic Places.

Ray Kroc finally got his hands on the stand in 1990 when McDonald’s Corp. acquired it. The building suffered damage in the 1994 Northridge earthquake and the restaurant was closed. It was subsequently scheduled for demolition but preservationists, including the Los Angeles Conservancy, fought to save the landmark. It was eventually restored.

The location has struggled with profitability and the Downey Planning Commission recently allowed the addition of a drive-thru at the location. The work is now complete and it's actually hard to see since it's routed along the backside of the building.

Image: LA Times

In 1959, the original neon sign was replaced with a a 60-foot golden arch with the original mascot Speedee the chef. Despite the creepy clown replacing him in the 1960's, Speedee is still running in Downey. 

Don't let the cute building fool you. 
More about McDonald's here.