The 30-year-old built an eight-minute lead over his main rivals starting the marathon run and was never challenged as he finished in eight hours, 14 mins. 18 seconds.
American Ben Hoffman was second in 8:19:23, with another German Jan Frodeno, the 2008 Olympic champion, third in 8:20:32.
"I never thought I was going to win this thing," Kienle told the crowd at the finish line, revealing that he had quit a training session three weeks ago due to problems with a troublesome Achilles tendon.
Australian Carfrae, meanwhile, used a brilliant run to successfully defend her title, overcoming a deficit of more than 14 minutes in the marathon and mowing down Swiss athlete Daniela Ryf.
Carfrea was in eighth place starting the run but powered home to win her third world title in 9:00:55 ahead of Ryf in second with Britain's Rachel Joyce third.
The ironman comprises a 2.4-mile (3.86km) swim, 112-mile (180.25km) bike and 26.2-mile (42.2km) run.
(Reporting By Andrew Both in Cary, North Carolina, editing by Nick Mulvenney)
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