Chen (11-3), who has won his past four starts, scattered five singles, walked one and struck out three. His 11 wins ties him for third-most in American League. On the road, Chen is 6-1 with a 3.94 ERA in nine starts.
Darren O'Day finished with a routine ninth inning.
It was Baltimore's eighth shutout. The Mariners have been shut out 12 times, tying Tampa Bay for the most in the American League.
The Orioles began their four-run third with singles by John Flaherty, Caleb Joseph and Nick Markakis.
Markakis' single drove in one run and on the next pitch Young hit his fourth homer. It was the Orioles' 123rd of the season, tying Toronto for the most in the majors.
Hisashi Iwakuma (8-5), who had won his previous three decisions, lost for the first time since June 25. He went seven innings, allowing four runs and seven hits. He struck out five without a walk.
Chen's only hint of trouble came in the first inning. James Jones walked then stole second, the rookie's 20 of the season.
After a groundout, Robinson Cano dribbled a ball in front of the plate. Chen flipped it with his glove to catcher Joseph, who tagged Jones as he slid in head first.
Mariners manager Lloyd McClendon protested the call but it was crew chief Fieldin Culbreth who initiated a TV replay review to determine if Joseph illegally blocked the plate.
After a review of 3:45, the original call was confirmed.
Iwakuma, who had just two three-ball counts, has not walked a batter in a club-record five straight starts, a span of 35 2-3 innings. He is second in club history in walk-less innings behind Cliff Lee's 38, set in June 2010.
Iwakuma has 33 strikeouts against no walks in those five starts.
Dustin Ackley of the Mariners had a single in the second to give him a seven-game hitting streak and push his July average to .373.
NOTES: Beyond the Mariners' acquisition of DH Kendrys Morales from the Twins before the game, the club also added two players from Triple-A Tacoma to the roster. Jesus Montero was called up for a second stint and started at DH. Chris Taylor started at short in his first major league game and singled in the fifth. When he called his parents in Virginia Beach to tell them the news, Taylor said, "it was tough to hold it together. That moment is something you're never going to forget. Talking to them on the phone, telling them the news, hearing their voices, how excited they were, it's just an indescribable feeling." To make room, the club sent RHP Taijuan Walker back to Tacoma and placed INF Willie Bloomquist (right knee contusion) on the 15-day DL. . The Orioles acquired INF Jimmy Paredes from Kansas City for cash considerations. He will report to the minors.