The Filet-O-Fish was created in 1962 by McDonald's franchisee Lou Groen to attract customers on Fridays. Groen operated in a neighborhood with many Catholic customers who traditionally avoided meat on Fridays, so he developed a fish sandwich for that market.
McDonald's tested the Filet-O-Fish alongside a competing meatless promotion before selecting Groen's sandwich. It became a permanent menu item and an important example of McDonald's adapting its product branding to local customer habits.
The sandwich is sometimes described as a nationwide launch from the beginning, but its origin was a franchise-level response in Cincinnati. Its later success turned a local solution into a familiar international McDonald's product.