Saunders stroked a two-run double in the fourth inning and added another run-scoring two-bagger in the sixth for the Mariners, who evened their season-ending three-game series.
Kyle Seager went 2-for-4 with a solo homer for the M's.
Iwakuma (9-5) scattered six hits, issued a single walk and struck out seven in winning his third consecutive start.
Dan Haren (12-13) was tagged for five runs (three earned) on seven hits while fanning eight in six full innings for the Angels.
Howie Kendrick went 3-for-5 with an RBI in the loss.
American League batting title contender Mike Trout went 1-for-5 for Los Angeles, lowering his average to .324 and likely ending his chances of catching triple-crown hopeful Miguel Cabrera of Detroit (.331).