Happ (10-11) became the fifth Blue Jays starter to reach double numbers in wins this season as the left-hander allowed eight hits, including a home run by Kyle Seager, and two runs.
It is the first time the Blue Jays (79-77) have had five starters win at least 10 games in one season. It is the third time they have had five pitchers with at least 10 wins when relievers are included.
James Paxton (6-4) did not survive the third inning in his first major-league game against the Blue Jays. He allowed seven hits, six walks and nine runs (eight earned) in 2 2-3 innings.
Paxton, who is from Ladner, B.C., was a first-round pick by Toronto in the 2009 draft. A deal could not be reached and Paxton was picked in the fourth round in the 2010 draft by the Mariners.
The Blue Jays led 4-1 after the first inning and 9-1 after three innings to hand Seattle (83-73) its third loss in a row.
The Mariners scored a run in the first when Robinson Cano doubled home Austin Jackson led off with an infield single.
The Blue Jays scored four in their first on a bases-loaded triple by Valencia with one out and a two-out single by Steve Tolleson. Toronto loaded the bases with one out on a single by Bautista and walks by Edwin Encarnacion, a wild pitch, and a walk by Dioner Navarro.
That set up Valencia's triple, which was not without some controversy. He was given the triple when a video review determined he was safe after the Blue Jays challenged the out call.
Toronto chased Paxton in the five-run third to increase their lead to 9-1. Valencia led off with a double. John Mayberry walked and Tolleson sacrificed the runners to second and third. Pillar then walked with two out to load the bases.
Jose Reyes singled off the glove of third baseman Seager to drive in a run, followed by Bautista walking to force in a run. Right-hander Brandon Maurer took over for Seattle.
A passed ball scored Pillar and Encarnacion scored two runs on a single deflected by Seager.
The Blue Jays scored an unearned run in the fourth on singles by Tolleson and Dalton Pompey, a native of Mississauga, Ont., and a throwing error by shortstop Chris Taylor.
Bautista hit his 35th homer of the season to lead off the fifth against Tom Wilhelmsen. Bautista moved past George Bell and tied Joe Carter for third on the Blue Jays' all-time list with 203 homers.
Seager hit his 25th homer of the season in the sixth to cut the lead to 11-2.
Pillar hit his first homer of the season, with Pompey on first after an infield single, against left-hander Lucas Luetge in the sixth.
Gose, who entered the game among the seventh-inning defensive changes, hit his second homer of the season in the seventh against right-hander Danny Farquhar.
Rookie right-hander Kendall Graveman allowed a run in the eighth.
Toronto reliever Sean Nolin allowed a homer to Chris Denorfia in the ninth.
NOTES: RHP Chris Young will not start for the Mariners in the finale of the four-game series on Thursday after going 0-3 with an 8.25 earned-run average in his past five starts. Manager Lloyd McClendon said he is showing fatigue after coming back this season from surgery. … DH Adam Lind (sore back) was out of the Blue Jays starting lineup for the third consecutive game but should return on Tuesday. …RHP R.A. Dickey (13-12, 3.82 ERA) will start for Toronto on Tuesday against Seattle RHP Felix Hernandez (14-5, 2.07 ERA). …Announced attendance at Rogers Centre was 15,548.