I heard it from a key grip, who heard it from a set designer, who heard it from the 2nd unit director of “Cannonball Run”, who said it was told to him by Ricardo Montalban one night over dinner with friends.
This is a true story.
Actor Roddy McDowell, Montalban’s friend and sometimes co-worker, spent much of the 1960s and 1970s dressed up like a man-sized chimpanzee. First as Cornelius in the original “Planet of the Apes”, then in later sequels as other ape characters. Unfortunately the elaborate makeup took forever to remove, so Roddy was often last to the catering table, and forced to live with whatever he could cobble together. So he usually just took some romaine lettuce, and mixed it with lemon, olive oil, some egg, and worcestershire sauce. This light concoction had the added benefit of not staining his ape makeup, so he could eat without doffing the mask. He would later perfect his recipe with black pepper and croutons.
Over the course of his ape-career it became a regular staple. By the time filming began on “Conquest of the Planet of the Apes”, the catering crew made it special for him. When other actors asked what he was having, the caterers named it after his character’s name: “It’s Caesar’s Salad.” The rest is history.