Hernandez (8-1) won his fifth straight start to match a career high. The Mariners broke open a game of odd bounces with a four-run seventh inning, and Seager hit a three-run homer in the ninth.
Robinson Cano was again relentlessly booed by Yankees fans, this time in a makeup of a game washed out April 30. In the lineup after sitting four straight with a bruised left hand, he went 1 for 3 with two walks. The former New York second baseman drew a mocking cheer when he was caught too far off first by David Phelps (1-3).
Seattle, which won games at Yankee Stadium on April 29 and May 1, took all three games in New York for the first time since sweeping three in 2002. The Mariners have won three straight overall.
Seager became the first major leaguer to hit two triples and at least one homer and one double since Hal Breeden for Montreal in 1973, STATS said. The last AL player to do it was Hoot Evers for Detroit in 1950.
Seager scored three runs and drove in three. One of his triples was the result of a rare mental mistake by Yankees shortstop Derek Jeter.
Seager hit a blooper down the line that bounced off the glove of sliding left fielder Brett Gardner. Jeter was also giving chase and had a chance to make the catch off the deflection, but couldn't come up with it.
With his back to third base umpire Brian Gorman, Jeter did not see the fair signal and he corralled the ball in foul territory and continued to jog several steps toward the corner. He looked surprised when he turned, and his throw to third was too late.
Phelps hollered "throw the ball!" as Seager headed to third. After the play was over, Jeter could be seen on television saying "I thought it was foul" as he walked back to the infield.
The Yankees missed Hernandez in April because of the rainout and they certainly wished they had not faced the Mariners' ace Monday. King Felix has been on quite a roll.
The 2010 AL Cy Young Award winner has not lost in nine starts dating to April 21. He was touched for two runs in the fourth when he kicked out his foot and redirected Ichiro Suzuki's grounder up the middle for a two-run single.
Hernandez gave up eight hits and two runs in seven innings. He struck out eight without allowing a walk. Hernandez last won five starts in a row in 2009.
Seager homered with two outs off Alfredo Aceves in the ninth three batters after Michael Saunders hit a home run.
Brad Miller had a two-run single with the bases loaded in the seventh to give Seattle a 4-2 lead and chase Phelps. Saunders added a two-run single off Matt Thornton.
NOTES: Yankees RF Carlos Beltran (right elbow bone spurs) went 0 for 3 as the designated hitter in an extended spring training game against the Phillies in Clearwater, Florida. Beltran said elbow felt good. ... Yankees C Francisco Cervelli (right hamstring strain) went 1 for 2 with a double and walk in his third extended spring training game. ... The Mariners will fly about 6,000 miles on this three-city road trip. ... Triple Crown hopeful California Chrome's Jockey, Victor Espinoza, and Alan Sherman, the son of 77-year-old trainer Art Sherman and his top assistant, threw out ceremonial first pitches.