Seattle's Jesus Montero hit a two-run homer in the second inning and Stefen Romero followed with another shot. With two out in the third, Nick Franklin's pop fly eluded the Dodgers' fielders for a double, and Justin Smoak followed with a two-run homer. Beckett allowed six hits and five earned runs.
Beckett tried to work on locating his fastball - in to left-handed batters and out to right-handers.
''I didn't know their guys ... were in our meeting before this game,'' Beckett joked. ''They figured it out pretty quick.''
A poor outing in spring often leads to talk about how a pitcher isn't concerned about results, but is trying to tweak his repertoire of pitches.
''Everybody says that during spring training, but you obviously want to have good results,'' Beckett said. ''You don't want to pitch badly. .. Balls were leaking back to the middle of the plate. I guess I need to work on that. That's sort of what I wanted to work on, and it didn't go good.''
Montero later hit a solo homer.
Joc Pederson hit a two-run homer for Los Angeles.
The game was a sellout, with an announced crowd of 13,427.
Mariners starter Blake Beavan gave up three runs, but only one earned, in three innings.
''Every outing is a learning experience for him,'' said Mariners manager Lloyd McClendon, who indicated Beavan made a poor pitch to Scott Van Slyke on Van Slyke's run-scoring single in the first.
Beavan also made a throwing error on a pickoff try while Montero made a fielding error.
''It was not all his fault,'' McClendon said about Beavan, who could be in the mix for a starting spot.
With his two homers, Montero raised his spring average to .350.
High expectations have yet to be met for Montero, who hit .208 with three homers and nine RBIs last season.
In addition, recent times have been tumultuous. He took a 50-game suspension last year in connection with the Biogenisis scandal and then reported to spring training overweight.
''He got off to a slow start,'' McClendon said. ''The last couple days he's swinging better.''
Dodgers outfielder Yasiel Puig was excused from Saturday's activities to deal with a personal matter, manager Don Mattingly said.
''It hasn't been a big deal,'' said Mattingly, who said Puig would be in the lineup on Sunday. ''I've known about it since the beginning of camp. No big deal.''
Mattingly declined to say whether Puig had to travel out of state.
''It's his deal. If he wants to discuss it he can.''
The Dodgers' injured stars, pitcher Zach Greinke and outfielder Matt Kemp, worked out Saturday morning.
Kemp took batting practice off Greinke and did some light base-running.
Greinke, between throwing a bullpen session and throwing off a practice-field mound to Kemp and others, estimated he threw about 85 pitches.
Of his strained calf, Greinke said: ''It's not 100 percent, but it should be in a couple days.''
Kemp is coming back from offseason ankle surgery. Neither is expected to make the trip to Australia to open the regular season March 22-23.
Greinke worked on several pitches, and judged his change-up to be especially effective.
''Matt hadn't seen pitches much,'' Greinke said. ''Once a couple weeks go by, he'll be taking better swings on that pitch. But he was hitting the fastball good.''
Greinke declined to say whether his next pitching session will be in a Cactus League game. But he indicated he felt well enough that he expects to try fielding practice within a few days.