Watson edged Chang 7-5, 5-7, 7-6 (7-4). Both players had seven breaks, won 55 percent of the points on serve and each took 129 points on the day.
Chang seemed on the verge of her own initial WTA title but Watson fought off four championship points when Chang was serving at 5-4 of the third set.
"After I saved the first one, I just took it point by point," said Watson. "I'm really proud of myself for getting through that."
Watson is the first British woman to win a WTA title since Sara Gomer was the champion in 1988 in California.
Watson's first-place check was for $37,000 while Chang collected $19,000 as the runner-up.
Swim Week Miami Beach 2014 [PHOTOS]