SEATTLE —The “Let’s go Blue Jays!” chants began just before the first pitch of Sunday’s series finale. It didn’t take long for the Blue Jays to give their (many, many) fans something to cheer about at Safeco Field.

Kevin Pillar led off the game with a double off Mariners left-hander James Paxton, and Josh Donaldson followed with an opposite-field home run to right field, kick-starting the Blue Jays’ 4-0 victory over the Mariners and giving Toronto a series win in Seattle.

Of the 41,137 fans in attendance on a pleasant Sunday afternoon, some 70 percent of them were Blue Jays fans — continuing the Canadians’ annual takeover of Safeco Field.

Donaldson finished with three hits and three RBI, and former Mariners left-hander J.A. Happ picked up his first win of the season with eight strikeouts over six scoreless innings for Toronto.

“It’s strange, sitting in the dugout and the crowd is always cheering, whether it’s one team or the other team,” Happ said. “It was an awesome atmosphere. It’s a lot of fun.”

The loss is a sour ending to an otherwise uplifting homestand for the Mariners, who finished 7-4 in their longest homestand of the season. They begin a weeklong road trip Monday in Minnesota.

Paxton (5-1) was hit with his first loss of the season. He threw a season-low four innings, allowing four earned runs on eight hits, with three walks and three strikeouts on 94 pitches.

“I just didn’t feel great today,” Paxton said. “I had a hard time finding my timing, my rhythm. Just didn’t have it.”

In the first inning, sitting on a 2-0 count, Donaldson was looking for a fastball — and got a 95-mph fastball on the outside corner. Paxton thought it was actually a pretty good pitch, but Donaldson sent it into the stands in right field to give the Blue Jays a 2-0 lead.

“I was definitely (looking) heater at that point because I know he’s got a really good heater, as he should,” Donaldson said. “I know he likes to go to it. He’s blown me up with it a few times in the past.”

Donaldson’s RBI single in the fourth scored Pillar to make it 3-0. Former Mariner Kendrys Morales added a run-scoring single later in the inning to make it 4-0.

Paxton spent most of May on the disabled list, and in three starts since returning to the rotation he’s allowed seven runs in 141/3 innings (a 4.40) earned-run average.

“I worked really hard to get back and kind of had that foot on the gas pedal to get back and make that first start. I feel like I may just be a little bit drained after working so hard to get back,” he said. “But like I said, we’ve got a plan to move forward and get me back to where I need to be.”

The Mariners were shut out for the sixth time this season, first since May 27 at Boston.

Happ came into Sunday 0-4 with a 5.33 ERA. The Mariners had chances early — putting multiple runners on base in the first, third and fourth innings — but couldn’t break through.

The Mariners were 0 for 10 with runners in scoring position on Sunday, and finished the season series with a 1-6 record against Toronto.

“We had a lot of chances in the first four innings but just couldn’t quite get that big hit to get us on the board,” Mariners manager Servais said. “For whatever reason, the Blue Jays have pitched our offensive players as tough as anybody all year. We just haven’t been able to get a lot going against them, for whatever reason.”

Mariners right-hander Emilio Pagan relieved Paxton and was brilliant over four innings. He didn’t allow a hit, walking one batter and striking out five.