Dodgers starter Josh Beckett was staked to a three-run lead in the first inning, which began against Seattle starter Blake Beavan with an infield single and stolen base by Dee Gordon, who took third on Beavan's wild pickoff attempt at second.
Carl Crawford walked, Scott Van Slyke singled home Gordon, and Crawford and Van Slyke scored on a ball hit by Clint Robinson that was ruled an error on first baseman Montero.
The Mariners tied the game in the top of the second inning off Beckett. After Smoak's leadoff walk, Montero and Romero slugged back-to-back homers.
Smoak broke the tie in the third inning with a homer to right field after Nick Franklin's two-out single. Seattle padded the lead with a run off Jamey Wright in the fourth inning.
In the eighth, Montero slugged his second home run of the night, a solo shot against Chris Withrow.
Beckett left after three innings. After the game, he said he was burned on the very fastballs he came into the game hoping to refine.
"The game plan was to throw heaters. I didn't know their hitters were in our meeting," Beckett said. "I struggled with the fastball to their extension side."
He said he didn't consider it a step back because "health-wise, I feel good, that's the biggest thing for me now. But definitely have things to work on."
Jose Dominguez, who followed Wright's two innings, had the most impressive outing of the night, a five-pitch scoreless sixth inning.
Up next:Felix Hernandez makes his second start of the spring Sunday as the Mariners host the Rangers at 1:05 p.m. PT at Peoria Sports Complex (listen on Gameday Audio). The Mariners' ace threw two scoreless innings in his Cactus League debut Tuesday against the Dodgers and is on schedule for his club-record seventh Opening Day start on March 31. Second baseman Robinson Cano hasn't played since undergoing root canal surgery Wednesday and his status remains uncertain for Sunday, but Corey Hart is expected to get his second outfield start of the season as he works back cautiously after missing last year following two knee surgeries.