The Dash now have five goals, most of any team in the tournament.
Mewis scored in the 12th minute, taking a sweet back heel pass from Groom and blasting the ball from just inside the penalty area.
Groom scored on a precisely executed leaping header in the 54th minute. It was her second goal of the tournament.
Rachel Daly scored two goals in Houston’s opening game, a 3-3 draw with the Utah Royals. She was listed as questionable for the game against the Reign, but started.
The Reign were coming off a scoreless draw against Sky Blue in its opening game. They have yet to score a goal in the tournament.
Former Reign coach Vlatko Andonovski, who now coaches the U.S. women’s national team, was at Zions Back Stadium for the game.
The Royals defeated Sky Blue 1-0 earlier Saturday on Amy Rodriguez’s goal late in the first half.