Robinson Cano and Hideki Matsui added two-run hits against Bedard (3-3), who gave up a career-high nine runs and lost a series opener at Yankee Stadium for the second time in three weeks.
Seattle, a team with high expectations coming into the season, has shown consistently low energy and lost 17 of its past 22 games. The Mariners are an AL-worst 18-31, and no team that fell more than 16 games under .500 has ever reached the postseason.
Mariners manager John McLaren was ejected by plate umpire Mike DiMuro in the second inning for arguing after Ichiro Suzuki took a called third strike.
Yankees manager Joe Girardi was missing, too, from this matchup of last-place teams. He served a one-game suspension as the result of an argument the previous night when he kicked his hat and inadvertently sent dirt onto an umpire.
Bench coach Rob Thomson finally got a win after leading the Yankees to an 0-2 record when Girardi was sick in the season's first week.
New York, which swept three games from Seattle from May 2-4, has outscored the Mariners 32-6 in going 4-0 against Seattle this season. Coming off two wins against Baltimore, the Yankees have stretched a winning streak to three for the fourth time this season - New York has not won four in a row.
Pettitte (4-5) had gone 0-4 in his five starts - all New York losses - since April 20, and he gave up multiple runs in either the fourth or fifth innings in each of them. In this one, he allowed two runs and eight hits in six innings, yielding a run-scoring single to Yuniesky Betancourt in the second and an RBI double to Adrian Beltre in the third. His strikeout total was his highest since fanning 10 on Aug. 21, 2006, for Houston at Cincinnati.
After the Yankees fell behind, the right-handed-hitting Duncan homered in the second, his first of the season. Then, with the Yankees clinging to a 3-2 lead in the fourth, Jason Giambi hit an opposite-field double to left, setting up Cano's two-run single, also to the opposite field.
Bedard allowed Matsui's two-run single in the fifth that made it 7-2. Sean Green relieved, struck out Duncan and gave up four singles around a walk.
