Spinners Amy Smith and Molly Strano pulled South Australia back from a strong position
It came as the pair turned the Blundstone Arena decider on its head, coming to the wicket 1 for 40 in pursuit of 8 for 242 before reaching the target with 14 balls to spare.
Perennial battlers in their first 10 years, Tasmania had only finished outside of the bottom two twice before last year.
But with a squad that included Villani, Molly Strano, Sarah Coyte, Naomi Stalenberg and Rachel Priest, they became the dominant team of this summer’s competition.
The final loomed as their biggest challenge, with South Australia in control at 2 for 208 in the 45th over after they won the toss and batted first.
Strano also took 3 for 39 in a tight spell late, while Smith finished with 3 for 33.
Then it was Villani and Manix-Geeves’ time to shine. The 21-year-old Manix-Geeves drove nicely through the covers, hitting eight boundaries in her 133-ball knock.
Villani was dominant through the offside, cutting and working the ball behind square.
She won the match in that fashion, cutting Brooke Harris to the boundary to spark large celebrations.
The win denied South Australia their second title, after lifting the trophy previously in 2015-16 when they ended New South Wales’ long run of 10 titles.