CHICAGO — In front of a packed home crowd at Wintrust Arena, the Chicago Sky defeated the Phoenix Mercury, 80-74, in Game 4 of the W.N.B.A. finals, giving the franchise its first championship.

It was the second W.N.B.A. title for Sky forward Candace Parker, who signed with the team as a free agent in February.

The sixth-seeded Sky were unlikely champions after finishing the regular season with a .500 record. They had to win two single-elimination games to reach the semifinals, where they knocked off the top-seeded Connecticut Suns and their star forward Jonquel Jones, who won the regular-season Most Valuable Player Award, to clinch a finals berth.

Parker, 35, is one of the most decorated basketball players ever. She won two N.C.A.A. championships at the University of Tennessee, and won her first W.N.B.A. championship in 2016 with the Los Angeles Sparks, the team she spent the first 13 years of her pro career with. She was finals M.V.P. that year and has two regular-season M.V.P. Awards. This year, she became the first woman featured on the cover of NBA 2k, the popular basketball video game.

The only thing she hadn’t done was, in part, why she came to Chicago: bring a championship to her hometown.

“Going home has given me a sense of peace,” Parker said in an Instagram post in February. She grew up in Naperville, a suburb of Chicago. “I believe things come full circle and I find myself playing professionally in the very place where I first picked up a basketball.”

Her arrival in Chicago put the Sky into title contention, even if an inconsistent season had many questioning how far they could go in the playoffs. That’s when Chicago relied on key performances from its other stars and role players.

Guard Diamond DeShields shifted to coming off the bench from starting late in the regular season, but Parker said after Game 3 that the team depends on her athleticism. DeShields was an essential part of the Sky’s dominant defensive effort in their 86-50 win over Phoenix in that game.

“Defense is something you’ve got to want to do,” DeShields said after Game 3, in which Chicago held the Mercury to 25.8 percent shooting. “I knew coming into tonight that was what I wanted to do. I wanted to guard everything. I wanted to guard everybody.”

Point guard Courtney Vandersloot, who led the league in assists for the fifth straight year, averaged 9.7 assists per game in this year’s playoffs entering Game 4. She logged a triple double in a double-overtime win over Connecticut — the league’s best defense in the regular season — in the first game of the semifinals, joining the Hall of Famer Sheryl Swoopes as the only players in W.N.B.A. history with a triple-double in the playoffs.

Other Sky players have come through in big ways, too.

Vandersloot’s wife, Sky guard Allie Quigley, was averaging 14 points per game in the playoffs before Sunday, the second most on the team. The top scorer has been guard-forward Kahleah Copper, who was averaging 18.6 points per game.

Sunday’s title culminated a remarkable rise for Copper, the No. 7 overall draft pick by the Washington Mystics in 2016. Copper was traded to Chicago ahead of the 2017 season in a deal that sent Elena Delle Donne, the Sky’s former superstar forward, to the Mystics. Copper started in just 12 games from 2017 to 2019, fading behind Vandersloot, Quigley and DeShields.

She had a breakout season in 2020 as she shifted to a starting role, more than doubling her minutes per game — to 31.3 in 2020 from 14.8 in 2019 — and points per game — to 14.9 from 6.7. Copper has since established herself as a star on the Sky and was the difference in this championship series.

“You have players that work, but you have players like Kah that really, really work, and you see it right way,” Chicago Coach James Wade said after Copper scored a game-high 22 points in Game 3.

It’s a disappointing finish to the season for the Mercury, who last won a championship in 2014, when they swept the Sky for their third franchise title.

Phoenix had a strong season behind Diana Taurasi, the W.N.B.A.’s career scoring leader, Brittney Griner, one of the best centers in the league, and Skylar Diggins-Smith, who has been a fearless scorer and playmaker her entire career.

But though they had kept Phoenix alive in so many critical games up to this point, on Sunday they couldn’t do enough to extend their season for one more game.