The Mariners trailed by two heading into the ninth, but staged a dramatic comeback to win their fourth in a row. The Blue Jays have dropped four straight.
Saunders hit a one-out, solo homer in the ninth to cut the Seattle deficit to 5-4 and the Toronto defense then collapsed while allowing the tying run.
Pinch-hitter Kyle Seager reached when third baseman Brett Lawrie threw wildly to first on a play that should have produced the final out. Munenori Kawasaki came on to run for Seager and moved around to third when a pickoff attempt by catcher J.P. Arencibia sailed wide.
A single by pinch-hitter John Jaso brought home Kawasaki with the tying run.
Charlie Furbush (2-1), who retired the only two men he faced in the ninth, collected the victory. He was the seventh of eight pitchers for Seattle with Brandon League coming on in the bottom of the 10th in a non-save situation.