Iwakuma notched his third straight start without allowing an earned run, taming the Astros on four hits. The right-hander issued one walk and struck out eight en route lowering his ERA to 1.79, second-best in the American League.
Carter Capps and Charlie Furbush combined for a scoreless eighth inning and Tom Wilhelmsen picked up his 16th save despite yielding a run in the ninth as the Mariners took the first of a three-game series.
Astros starter Dallas Keuchel (3-3) absorbed the loss, allowing two runs (one earned) on seven hits over six frames.
Jose Altuve and Carlos Corporan each drove in a run for Houston.