Penny Taylor posted 16 points, 13 boards and seven assists, while Diana Taurasi added 17 points, six rebounds and five helpers for the Mercury, who won three of four meetings against the Lynx during the regular season.
Phoenix swept Los Angeles in the conference semifinals.
Lindsay Whalen tallied 25 points, but WNBA MVP Maya Moore was limited to nine points on 3-of-9 shooting for defending-champion Minnesota, which swept San Antonio in two games in the conference semis.
The Mercury opened up a 42-31 lead at halftime and blew the game open in the third following a 14-0 run.
Taylor netted six points during the burst, which was capped by Taurasi's jumper to give Phoenix a 63-39 cushion.
The Mercury led by as many as 25 in the frame before settling for a 67-47 advantage heading into the fourth, where they cruised to the easy win.
Game 2 is Sunday in Minneapolis.