Cashner allowed one run and four hits in 6 1-3 innings. The 6-foot-6 right-hander struck out six and walked one.
"If you were to write up a game for us, it would feature outstanding starting pitching and maybe a home run or two with the type of power we have in our lineup," Padres manager Bud Black said.
"This was a game you could maybe say could be along the lines of game you might see during the regular season."
Myers went deep on J.A. Happ's first pitch of the game. Kemp also homered in San Diego's three-run first.
Dustin Ackley had two hits for the Mariners, including a two-run homer.
"He swung the bat very well," manager Lloyd McClendon said. "I was very pleased."
Mariners catcher Mike Zunino also went 2 for 3.
Mariners: J.A. Happ allowed six runs, five earned, and 11 hits in five innings. He walked two and struck out three.
"As the ballgame went on, I thought he started to throw better," McClendon said. "It's something to build on. Got his pitch count up over 80. The next time out I expect he'll be better."
Happ said he was too tentative early on, but finished strong.
Padres: Black said Cashner pounded the strike zone. "A good all-round day," Black said.
TRIBUTE FOR SANCHEZ
The teams observed a moment of silence for Victor Sanchez, a pitching prospect for Seattle who died as a result of injuries suffered from being hit by a boat propeller while swimming at a beach in the Venezuelan coastal town of Campano.
"Victor was a tremendous young man and a wonderful teammate," said Jack Zduriencik, the Mariners' executive vice present and general manager. "He was a very talented player who was close to fulfilling his promise as a major leaguer."
MYERS SHOWS VERSATILITY
Myers went 2 for 4, and the outfielder also got some work at first base.
"This guy is an athlete," Black said. "He has bounced around the diamond professionally, catcher, third base. He's played first base. He's played the outfield.
"This guy is 24 years old with a young, enthusiastic heart to play. It doesn't matter where he plays. He likes to play."
BLAME THAT UMP
McClendon had a prolonged discussion with Fernando Rodney when the manager went to pull the reliever from the game. McClendon wanted to limit Rodney to 16 pitches, but the manager thought he got to 18.
"It was Rodney being Rodney," McClendon said. "He wanted to tell me about how many the umpire missed. (I said), 'That's good, but it's time for you to go.'"
MARINERS CUT ROSTER
The Mariners made 10 moves to cut their roster to 28 players.
Pitchers Roenis Ellis and Carson Smith were optioned to Triple-A Tacoma. Pitchers Joe Saunders, Kevin Correia, Mark Low and Jordan Pries were reassigned to minor league camp, along with infielders Shawn O'Malley and Carlos Rivero, catcher John Baker and outfielder Endy Chavez.
LAST ROTATION SPOT OPEN
The competition for the final spot in San Diego's rotation is coming down to the wire.
"We'll watch them closely," Black said, although, "I don't think it comes down to one game."
Of the two, Despaigne probably is better suited to a long reliever role, Black said.
"If one of them does filter back into the bullpen, if we make that decision, I would think that Despaigne suits that role a little better than Brandon," he said.
"I think Odrisamer truly is a guy who can pitch multiple innings."
Mariners: Outfielder Alex Jackson was a late scratch because of shoulder soreness.
Padres: Infielder Yonder Alonso missed the game with an illness.
Mariners: Taijuan Walker, who has thrown 18 scoreless innings, will start Monday at home vs. the Los Angeles Angels.
Padres: Ian Kennedy will start Monday against Cincinnati.