Brands Hatch is familiar territory for the 21-year-old, who last year at the southern circuit became the first woman to win a round of the British Formula Three championship.
Winning the W Series secures Chadwick a $500,000 prize payout, created to aid her progression in the next level of motorsport.
The payday, more than the annual wages of some current grand prix drivers, and the publicity could boost her hopes of eventually becoming the first woman since 1976 (Italian Lella Lombardi) to race in Formula One.
Chadwick blasted off the grid at the start, leading Alice Powell and Assen victor Emma Kimilainen in the first half of the race.
Beitske Visser was the Brit's chief rival in the title fight, but a third-place finish at Brands Hatch saw her end the season 10 points behind Chadwick.
"I'm under no illusions as to how tough it's going to get", she said after clinching the crown with fourth place in the season's final race. She managed it, taking the final podium position with just three laps to go.
Oxfordshire-born Alice Powell took her first win in the W Series, having pressured her way past Chadwick from second on the grid.
'There's no certainty as to whether I'll be back, ' she said. At the beginning of the year I thought it just might be a great season.
'I'm relieved. It feels wonderful to win the championship.
Finishing in the top 12 for the overall championship is significant, as those drivers receive an automatic invitation to return for the 2020 W Series competition. "It hits home to me that, if I do a good job and get to be where I need to be and there is an opportunity down the line, it makes that dream feel a lot more like reality".
"I suspected that W Series had the potential to be something big, but I never expected it to be this big". Powell was third overall on 76.
"The crowd has been incredible today and that's a huge credit to the way W Series has promoted us drivers".
"I think we've done that now and next year has got to be all about the drivers and telling the drivers' stories and introducing the personalities and characters".