Seimone Augustus added 18 points to help make sure Minnesota will have the chance to win a fourth title in seven years.
"They challenged us in many ways that whoever comes out of the other series, Phoenix or LA, will challenge us in the same way," Augustus said.
The Lynx will play Los Angeles in a rematch of the WNBA Finals. The Sparks won last season's championship round in five games.
Despite winning all three semifinal games by double digits, the Lynx felt fortunate to escape Washington just as the Mystics seemed to be gaining their footing.
"This is a team that didn't have most of their group together for a big stretch of the season," Lynx coach Cheryl Reeve said of Washington. "What concerned me was the longer they played, the better they were getting. And so we thought it was really important not to give them that life."
Elena Delle Donne scored 15 points to lead the Mystics, who exit the playoffs after reaching the semifinals for the first time since 2002.
Tierra Ruffin-Pratt added 14 points and Kristi Toliver (Maryland) had 13 for Washington, which forced the Lynx into a series-high 13 turnovers.
But the Mystics scored only 31 second-half points in their sixth loss in as many games against Minnesota in 2017, including regular season and playoffs.
"When you're matched up against a team like Minnesota, that's the kind of team that sets the bar," Toliver said. "And we got to play against the bar."
Boxing: Gennady Golovkin retained his middleweight titles Saturday night, fighting to a 12-round draw with Canelo Alvarez. One judge had Alvarez winning 118-110, a second had it 115-113 in Golovkin's favor while the third had it 114-114. The Associated Press scored it 114-114.
Golf: Marc Leishman closed with a birdie for a 4-under 67 to set the BMW Championship tournament record in Lake Forest, Ill. The victory sends him to the Tour Championship. ... Anna Nordqvist beat American Brittany Altomare at the first extra hole to win the Evian Championship in Evian-Les-Bains, France. ... Jerry Kelly beat Lee Janzen by one stroke to win the Champions Tour event in Victoria, British Columbia.
BASKETBALL: Slovenia won its first European championship, defeating Serbia, 93-85, in Istanbul behind 35 points from Goran Dragic. Spain got 26 points from Pau Gasol and beat Russia, 93-85, for third.
Women's college soccer: Caitlin Farrell and Meaghan Nally each scored to lead No. 18 Georgetown (6-2-1) to a 2-0 win over host Towson (3-5-2).
College field hockey: Freshman Kyler Greenwalt and senior Lein Holsboer each scored twice to lead No. 9 Maryland (5-2) to a 4-1 win over Bucknell (3-6).
Auto racing: Josef Newgarden won his first IndyCar championship by closing out the best year of his career with a second-place finish at Sonoma (Calif.) Raceway. . ... Lewis Hamilton won the Singapore Grand Prix to extend his Formula One championship lead to 28 points.