Filet-O-Fish was the McDonald's product name trademarked in 1962 after franchisee Lou Groen created the sandwich.
Groen operated a McDonald's restaurant in Cincinnati, where many nearby customers followed Catholic practices that included avoiding meat on Fridays, especially during Lent. He developed a fish sandwich to attract those customers at a time when McDonald's sales were heavily dependent on hamburgers.
The product was tested before its 1963 introduction, and its trademark history preceded the menu launch. That timing makes the fact easy to mix up with the sandwich's debut year. The name uses McDonald's “-O-” naming style, an approach that also appeared in other products and promotions.
The Filet-O-Fish became a permanent part of the menu and an important example of local franchise innovation influencing the national brand. Its origin also shows that McDonald's product branding was shaped not only by corporate marketing, but by regional customer behavior.