The world number 22 looked calm and focused during the 73-minute semi-final, while Muguruza appeared out of sorts and frustrated, clattering her racket on to the court in the second set.
"I was just called upon yesterday and I won yesterday". Wang trails Barty 0-1 in the pair's head-to-head record after falling in straight sets against the Australian in Strasbourg earlier this year.
Ash Barty has landed a big blow on Serena Williams after winning her third-career WTA title.
Barty stormed out of the blocks with two breaks to race into a 4-0 lead, with her Chinese opponent registering just six points as the 22-year-old dominated the early exchanges.
In the first semifinal of last year's tournament champion Julia Gorges in three sets lost to the world No. 19 Ashleigh Barty (6:4, 3:6, 2:6).
That solitary break was enough to seal her the title, Barty finishing things off with a delicately sliced backhand that spun out of Wang's reach.
Serena opted to end her 2018 season two weeks ago, allowing players like Barty to overtake her.
Barty only advanced to the final four after Caroline Garcia scored an upset victory over Aryna Sabalenka on Friday to see the Australian finish on top of her group with a one-game advantage following the round robin phase.
Sunday's win is her third WTA singles title, adding to her breakthrough 2017 trophy at the Malaysian Open and this year's grasscourt win in Nottingham.
Barty told her post-match press conference: "It's always nice to get better as a match goes on".
"It's been a phenomenal year and I'm looking forward to a bit of holiday and then getting ready for the Australian summer".
Remarkably she started 2017 ranked No.325 in the world to begin her tennis comeback.