Napoli's two-run homer capped a five-run first inning before belting a two-run homer in the sixth to give the Rangers some insurance. Texas hit four homers in the game, including three in the sixth inning.
Texas has won seven of its last eight games and has reached double figures in consecutive contests for the first time this season. The Mariners are 2-4 on their road trip.
The Rangers jumped out a 5-0 lead in the first against Yovani Gallardo (3-7) by putting together five straight hits with two outs. Rougned Odor started the scoring with an RBI double and Carlos Gomez followed with a two-run single. Napoli then hit his first homer of the game.
Seattle got back in the game with a run in the third off Martin Perez (3-6) and then three runs in the sixth. Mike Zunino's three-run homer cut the Texas lead to 5-4.
Texas blew the game open the bottom of the frame, scoring five times off Dan Altavilla on three homers. Odor homered to open the inning, Napoli hit his second blast two batters later and Shin-Soo Choo added another two-run homer to extend the lead to 10-4.
Perez picked up his first win since May 18 as he allowed four runs on seven hits in 5 1/3 innings. He struck out five and walked two.
Gallardo lasted just five innings, giving up five runs on six hits. He allowed just one hit after the first inning.
Texas had 10 hits, with Napoli, Odor, Choo and Nomar Mazara each collecting two. Gomez, Odor and Choo each had two RBIs.
Guillermo Heredia went 2-for-4 with a run scored for Seattle and Mitch Haniger also had two hits. Taylor Motter brought in Seattle's first run with an RBI double in the third inning.
NOTES: The Rangers put RHP Andrew Cashner on the 10-day disabled list with a strained left oblique. The club purchased the contract of RHP Ernesto Frieri to take Cashner's roster spot. ... Texas INF Hanser Alberto will have arthroscopic surgery on his right shoulder Monday. He's been on the disabled list all season because of the shoulder. ... Seattle acquired RHP Pat Light on waivers from Pittsburgh and assigned him to Triple-A Tacoma. ... Seattle recalled RHP Rob Whalen from Triple-A Tacoma and optioned LHP Zac Curtis to Double-A Arkansas.