The Filet-O-Fish was created in 1962 to appeal to customers who ate fish on Fridays.
McDonald's franchisee Lou Groen developed the sandwich in Cincinnati, where his restaurant served a large Catholic population. Sales traditionally weakened on Fridays because many Catholic customers avoided meat, so Groen proposed a fish-based alternative.
The sandwich originally used a breaded fish fillet, cheese, and tartar sauce on a bun. After testing competing meatless options, McDonald's selected the Filet-O-Fish and added it to the menu in 1962. It became a durable part of the chain's product branding.
The item is sometimes described as a seasonal or temporary promotion, but it became a permanent menu product. Its recipe and fish sourcing have changed in different periods and markets, while the name and basic concept have remained recognizable.