When Lee Iacocca dreamed up the Mustang while GM at Ford he would never have dreamed of the sales success his idea would become. It stormed out of the gates in terms of production success.
The initial launch featured a lot of broad media and advertising and an attractive base retail price of $2,320. Ford expected annual sales of around 100,000 units. That forecast was basically useless after the first day the Mustang went on sale when 22,000 Mustangs were ordered on that first day alone.
Initial 1964 sales top 121,000 units and the first full year (1965) saw sales take off even more with 559,500 units produced. The peak of the first generation Mustang sales was the 1966 model year with 607,568 units sold. The first generation had a number of changes and variants over those first initial years as Ford tried to capitalize on the initial success. The truth is that the Mustang got too fat and heavy and people lost interest.
Sales steadily fell in the subsequent generation years as Ford lost some of its magic and consumer tastes changed. Consumers were starting to look for more fuel-efficient cars that were cheap to insure and capable of passing newly introduced emissions standards. The muscle car era came to an end and Ford was still till heavily focused there with the Mustang and you can see it in the fall in 1st generation Mustang sales in latter years.