Stewart was the league MVP and was selected the Finals MVP after averaging 25.6 points in the three games. She scored 17 points in the first half as the Storm raced to a 47-30 lead.
Natasha Howard added career-high 29 points and 14 rebounds for the Storm and first-year coach Dan Hughes.
Sue Bird, also a member of a Seattle's championship teams in 2004 and 2010, had 10 points and 10 assists.
Elena Delle Donne scored 23 points for the Mystics. Kristi Toliver had 20 points.