The second-seeded Hewitt posted a 6-3, 6-4 triumph for his third title of the year and the 22nd of his career. He also won at Sydney and Rotterdam earlier this year. He lost to Andre Agassi two Sundays ago in the Cincinnati final.
Muller, the No. 1 junior player in 2001, had spoiled a potential heavyweight rematch between Hewitt and Agassi, beating the top-seeded American in Saturday's semifinals.
"I played the best tournament of my career and beat Andre Agassi," said Muller. "I have some regrets from today. I could have played better, but I can't have everything."
The 21-year-old Muller, who won a pair of Challenger series titles earlier this year, became the first player from Luxembourg to reach an ATP final. The big-serving lefty used 11 aces to beat Agassi.
Swim Week Miami Beach 2014 [PHOTOS]