The unseeded Dane had advanced on Saturday when top seed Andy Roddick withdrew from the tournament with an ankle injury, and he made the most of his opportunity against Mirnyi. He earned his third career title and his first since Tokyo in 2002 which was also his last appearance in a final.
A sprained left ankle forced Roddick to withdraw from his semi-final, and Carlsen was much fresher than Mirnyi, who had long matches all week and competed in doubles.
Mirnyi fell to 1-2 in finals appearances - all of which have come indoors.
Mirnyi upset second-seeded Tommy Haas of Germany, 7-5, 6-3 in the semi-finals.
Russia's Vera Zvonareva defended a title for the first time in her career here on Saturday.
Zvonareva defeated American third seed Meghann Shaughnessy 7-6 (7/3), 6-2 and did not drop a set in winning her second title in a row here. The 20-year-old denied Shaughnessy her first crown since 2003 in Canberra.
"I was really happy to be back in the finals," Zvonareva said. "I wasn't really thinking about the pressure of being the defending champion but I'm really happy to have won since this was the first time that I've been in this situation."