The Egg McMuffin became McDonald's first nationally available breakfast item in 1975.
The sandwich had first been introduced in 1971, after Herb Peterson developed it as a portable breakfast built around an English muffin, egg, Canadian bacon, and cheese. McDonald's then expanded the product beyond its initial test and regional availability, making it nationally available in 1975.
This rollout was a major branding milestone because it helped McDonald's become associated with breakfast as well as hamburgers. A signature morning product gave the company a reason to attract customers at a new time of day and supported later breakfast-focused advertising.
The introduction date and national-availability date are often confused. 1971 refers to the sandwich's initial introduction, while 1975 marks its nationwide availability. The McGriddle was introduced decades later, so it cannot be the answer.