Pegula, who turns 32 on Tuesday, became the second-oldest woman to win the tournament after her American compatriot Venus Williams, who triumphed in 2014 when she was 33.
"It's a great birthday present for me," Pegula said after ending a three-match losing streak in WTA 1000 finals.
"I get to celebrate by flying back home in a few hours. Super happy to go home with the trophy. It was a great week."
The world No.5 quickly built a 3-0 lead in the first set on Saturday night, as she kept Svitolina near the baseline before getting the first break with a backhand cross-court shot and holding her own serve with a masterful backhand slice.
The Ukrainian's attempt to bounce back ended when former US Open finalist Pegula saved two break points to go 5-1 up. A frustrated Svitolina, who had won all four of her previous WTA 1000 finals, netted a forehand as Pegula took the set.
Pegula was measured in the second set as she got a decisive break in the fifth game, and she won the match with an ace to secure her 10th title on the tour.
"Speechless after that fight," Pegula said.
"I was really trying to put myself out there, and I was playing like there is no tomorrow, just trying to put all the effort in."
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