The Impact won for the fourth time in five outings to increase their lead in the Eastern Conference to four points over idle New York.
Mertens took a through ball from Patrice Bernier in the 14th minute and beat goalkeeper Tally Hall. DiViao scored in the 32nd minute.
It was the MLS-leading ninth win of the season for Montreal, which has two games in hand over New York.
Houston (6-5-4) has seen its winless streak reach five games. The Dynamo had played to a draw in their previous two contests.