Iwakuma (1-3) took a shutout into the sixth inning and allowed four hits to end a career-worst, four-game losing game dating to last September. He walked two and struck out three.
Sonny Gray (3-3) allowed a career high-matching seven runs and 11 hits in his seven-plus innings. He walked one and struck out six in losing his second straight decision.
Gray, coming off the shortest start of his career, gave up Martin's two-run homer in the third.
Cano, who also doubled and scored, led off the fourth with his team-leading ninth homer. Seager drilled a three-run homer off Marc Rzepczynski in the eighth.
The A's loaded the bases with two outs in the second, but a nice play from Ketel Marte ended the threat. Marte went 2 for 4 and is hitting .370 against the A's. Steve Clevenger also had two hits for Seattle.
Mariners: RHP Tony Zych was placed on the 15-day disabled list with tendinitis in his rotator cuff. ... RHP Steve Johnson was recalled from Triple-A Tacoma to take his place. "He's just kind of had some nagging soreness in his shoulder, back and forth probably for the last week or 10 days," Mariners manager Scott Servais said. "That's why he hasn't been in there consistently."
Athletics: IF Danny Valencia (hamstring) went 0 for 3 in a rehab game Monday and was with Oakland taking batting practice and working with A's coach Ron Washington. He'll get a second rehab start Wednesday and will most likely rejoin the team on its road trip, which begins Friday in Baltimore.
Mariners: RHP Felix Hernandez (2-2, 1.38) makes the start Wednesday afternoon in search of his 146th career win, which would make him Seattle's all-time winningest pitcher. He's 11-3 in 22 games at the Coliseum.
Athletics: LHP Sean Manaea (0-0, 7.20) makes his second career start Wednesday against the Mariners.