Martinez, a former Mariners player, hit career homers No. 200 and 201 in New York's first appearance at Safeco Field. The Yankees pulled even with Texas and Cleveland for the top record in the AL at 65-42.
Roger Clemens (10-4) struggled early, but settled down in the middle innings. He allowed four runs and six hits in seven innings.
Martinez has three multihomer games this season and 13 in his career.
Chuck Knoblauch went 3-for-4 with a single, double and triple and scored three times for the Yankees. Williams was 4-for-5 with a double and a triple, one day after he homered twice and drove in five runs in New York's win over Toronto.
Seattle starter Jamie Moyer (10-6) didn't have his best stuff, allowing five runs on seven hits and two walks over six innings. It snapped a string of three strong starts where he gave up only four runs and 19 hits over 22 innings.
New York didn't waste any time getting on the board. After a game-opening walk to Knoblauch and a one-out single by Paul O'Neill, Williams hit a two-run triple to the wall in right-center.
The Mariners responded with two runs in the bottom half when Alex Rodriguez hit his 26th homer, a two-run shot that tied the game.
Seattle took a 4-3 lead in the third. Ken Griffey Jr. hit a bases-loaded RBI single, followed by a sacrifice fly by Rodriguez.
Martinez followed Jorge Posada's one-out double in the sixth with the homer that put the Yankees ahead for good.
Williams added an RBI single in the seventh and another single in the ninth.
Knoblauch opened the ninth with a triple and scored on O'Neill's single one out later to make it 7-4.
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