Saunders (7-8) yielded six hits, didn't issue a walk and struck out two in an efficient, 101-pitch performance.
Charlie Furbush tossed a scoreless eighth and Tom Wilhemlsen finished up with a 1-2-3 ninth for his 18th save as the Mariners claimed the rubber match of a three-game interleague set.
Nick Franklin socked a solo homer in the first, putting the M's ahead to stay, and Justin Smoak added a two-run shot in the third.
Reds starter Bronson Arroyo (7-7) was tagged with the loss, surrendering three runs on five hits over six innings.
Chris Heisey went 2-for-3 with an RBI for the third-place Reds, who came into the day trailing National League Central-leading Pittsburgh by 3 1/2 games.