The 23-time Grand Slam winner defeated Jessica Pegula in the final 6-3, 6-4, to claim her 73rd WTA title and first in three years.
Speaking after the match, Williams said: "It was a good match.
"I don't think I hit a tremendous amount of aces - she was getting all my serves back.
"My groundstrokes were probably the strongest thing in that match.
"It's good for me to win matches where I'm not getting too many free points, so I have to rely on my groundstrokes."
She added: "You have to be your biggest cheerleader, especially if you're feeling you're not doing everything right - or even if you are doing everything right, but things aren't working out for you."
The 38-year-old had not previously won a set in any of the finals she had reached since the birth of baby Alexis Olympia in September 2017.
She fell twice in the final round at Wimbledon and the US Open and retired in the first set of the 2019 Rogers Cup Final.
Williams had a complicated birth and Alexis had to be delivered by emergency C-section, as she was in distress.
The tennis star then developed blood clots in her lungs, causing her to cough, which ruptured her cesarean scar.
She had to have life saving surgery and several weeks of bed rest, delaying her return to training.
During her winner's speech Williams announced she would be donating her winnings to the Australian bushfire relief fund.
And the former No 1 said she would be auctioning off each dress she’s worn across her nine matches in Auckland.
In Australia, Karolina Pliskova completed a successful defence of her title at the Brisbane International with a 6-4, 4-6, 7-5 victory over Madison Keys.
Pliskova had to fight hard for her third career title.
Keys, finding herself 6-4 3-2 down, won four of the next five games to force a third set but eventually succumbed in a contest which lasted a little over two hours.