The Mariners were leading 3-2 in the fifth inning when Saunders' three-run blast broke the game open, and the Seattle center fielder capped the scoring in the ninth with a two-run shot.
Erasmo Ramirez (3-0) earned the decision, giving up four runs on four hits in a six-inning effort. He didn't issue a walk and struck out six.
Charlie Furbush hurled 1 2/3 scoreless relief innings and Danny Farquhar notched his first career save as the Mariners bounced back from an 11-8 loss in Friday's series opener.
Matt Wieters belted solo homer, finishing with two RBI, and Manny Machado also homered for the Orioles.