Laffey, 25, is 18-21 with a 4.41 ERA over parts of four Major League seasons. He was 2-3 with a 4.53 ERA with the Indians last year in 29 games, including five starts when he went 2-2 with a 3.60 ERA.
It's not certain whether the Mariners will look at him as a starting candidate or a reliever. He was battling for the fifth spot in the Indians' rotation.
Seattle has a fair number of candidates already battling for its final two rotation spots behind returners Felix Hernandez, Jason Vargas and Doug Fister. Veteran Erik Bedard and rookie prospect Michael Pineda appear to have strong chances of earning spots if they stay healthy, while Luke French, David Pauley and Nate Robertson are also contending.
The club doesn't have a lot of experienced lefties in the bullpen, however, with Garrett Olson the only returning left-handed reliever. Laffey has one Minor League option remaining, and the Mariners could send him to Triple-A.
Laffey has thrown one inning this spring for the Indians, giving up one hit and no runs. He's scheduled to join the Mariners on Thursday.
Mariners manager Eric Wedge and pitching coach Carl Willis both have worked with Laffey in the past with the Indians, where he was a 16th-round Draft pick in '03.
"With Aaron, we felt we had the opportunity to acquire a left-handed pitcher with Major League experience," general manager Jack Zduriencik said in a statement released by the club. "We will bring him to Spring Training and give him a chance to compete for a position on our pitching staff."
Lawson, 25, was one of the four players Seattle obtained last year from Texas in the Cliff Lee deal, along with Justin Smoak, Josh Lueke and Blake Beavan. He hit .293 with nine home runs, 56 RBIs and 75 runs in Double-A ball, splitting the year between Frisco and West Tenn.
Lawson was not in the Mariners' Major League camp.
To make room for Laffey on the 40-man roster, Shawn Kelley was placed on the 60-day disabled list. He's recovering from partial Tommy John surgery and isn't expected back until June.