The Egg McMuffin was McDonald's first national breakfast sandwich.
Herb Peterson created the sandwich in 1971 at a McDonald's in Santa Barbara, California. It used an English muffin, a round egg, Canadian bacon, and cheese, giving McDonald's a portable breakfast item that could be prepared in the restaurant.
The product became nationally available in 1975 and helped transform breakfast into a major part of McDonald's identity. Its name also extended the McDonald's naming style through the “Mc” prefix, making the product itself a lasting piece of the brand.
The Egg McMuffin is sometimes dated only to 1975 because that was its national rollout. Its origin was earlier: the sandwich was developed locally in 1971 before becoming a nationwide menu item.