From a Washington Post obit by Phil Davison headlined “Dan Wieden, adman who coined ‘Just Do It’ for Nike, dies at 77”:
The three-word, eight-letter tagline, along with Nike’s swoosh logo is considered one of the most successful and recognizable in the history of advertising, and it came about as the shoe company was struggling against formidable competitors such as Reebok and Adidas, whose ads capitalized on the youth-oriented fitness craze of the 1980s.