Barty entered the final 0-5 against Svitolina and the Ukraine had won her last nine matches at the WTA Finals.
Barty, who had never beaten Svitolina in five previous attempts, won 6-4, 6-3 in one hour and 26 minutes at the Shenzhen Bay Sports Centre.
Barty's income for a professional tennis play, man or woman, took her 2019 prize money haul to $16.37 million, trebling her entire career earnings to $23.87 million.
Svitolina gained an early break in the second when Barty double-faulted on break point, but the Australian responded by reeling off the next three games in style.
Barty rose to the top in June after winning the Nature Valley Classic in Birmingham in the United Kingdom, beating German tennis player Julia Goerges 6-3 7-5.
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"That whole match was at an incredibly high level", said Barty. "I think it was also important for me to move forward, then to try to bring Elina in a few times as well, which worked really well", she said.
With the win, Barty assured herself of the World No. 1 ranking to end the season and became the first Australian since Evonne Goolagong Cawley in 1976 to win the WTA Finals crown. An unforced error off this side gave Svitolina a game point to go 15-all, but the Australian managed to keep it together for a straightforward hold to go 2-1.
Throw in an equal-tour-high 56 wins from 68 matches and Barty is nothing less than a worthy year-end world No.1.
"It has been a year of incredible ups and downs, I think more ups and downs". The first person to consistently do so was Belinda Bencic in their semi-finals yesterday, and that was the first time Svitolina looked truly vulnerable, but eventually those long rallies ground Bencic down and she was forced to withdraw. She had lost just one set before the final to be on track to claim a maximum prize of $4.725 million before falling short in the decider. And of course she won her maiden Grand Slam at the French Open in June and captured Miami. "We're really excited to do it", Barty, who has four dogs, said.