The 21-year-old, ranked 127th in the world, was broken in her first service game by her more experienced Romanian opponent but rallied to become the third qualifier to win the Hobart title after Mona Barthel (2012) and Garbine Muguruza (2014).
Mertens had passed up the chance to qualify for the Australian Open singles draw by remaining in Hobart and was rewarded for her loyalty to the tournament when she wrapped up the title in 74 minutes.
World number 40 Niculescu will have little time to recover from missing out on a fourth career title before she opens her 10th Australian Open campaign against a qualifier in Melbourne on Tuesday.
(Reporting by Nick Mulvenney, editing by Peter Rutherford)
