The American, who will face China's Zhang Shuai in the first round at Melbourne Park, racked up 44 winners against Mertens, who was chasing a third title in Hobart after triumphs in 2017 and 2018, as well as a runners-up finish last year.
Ranked 67th in the world, Kessler wrapped up the win in just over two hours after a dominant deciding set, as she aims to improve on last year's second-round Australian Open exit.
Mertens faces Swiss Viktorija Golubic in her opening round at the Australian Open. The Belgian reached the semi-finals of the year's opening Grand Slam in 2018.