The 23-year-old Parry, ranked 50th in the world, held her nerve to seal a 6-4, 0-6, 6-3 win over the vastly more experienced Mertens, the world number 24.
Parry started confidently, clinching the first set after breaking Mertens for the second time in the 10th game of the opening set.
But Mertens, chasing her 11th singles title, came roaring back to race through the second set to love to regain the initiative.
Parry responded with an early break to take a 2-0 lead in the decisive third set, her powerful forehand continuing to cause problems for Mertens.
A further break gave Parry a 5-1 lead, and although she was broken immediately, she closed out the set to complete her first tournament win.