McDonald’s introduced the Filet-O-Fish sandwich nationally in 1963.
McDonald’s franchisee Lou Groen developed the sandwich in Cincinnati, Ohio, where many customers observed meatless religious meals on Fridays. Sales were especially weak on Fridays, so Groen proposed a fish sandwich to attract those customers.
The sandwich was first tested in the Cincinnati area in 1962. McDonald’s founder Ray Kroc initially preferred a meatless pineapple-cheese sandwich, but the Filet-O-Fish sold far better in the test. It then became a national menu item in 1963.
The Filet-O-Fish is commonly associated with Lent, although McDonald’s sells it year-round. The sandwich’s fish, cheese, tartar sauce, and bun have changed in details over time, and fish sourcing can vary by market.