The Rockies cleared space on the Major League roster by returning left-handed reliever Danny Rosenbaum to the Nationals -- the team from which the Rockies selected him in the Rule 5 Draft.
Garland, 33, gives a veteran presence to a Rockies staff that entered Spring Training young and with a limited number of candidates. Garland has not pitched in the Majors since 2011, when he went 1-5 with a 4.33 ERA for the Dodgers and battled shoulder issues. After missing the 2012 season following shoulder surgery, he signed a Minor League deal with the Mariners last month and proved that he is healthy by going 1-0 with a 2.25 ERA in four Cactus League starts.
When it became clear that he would not be on Seattle's Opening Day roster, he requested and received his unconditional release on Saturday. It didn't take long for the Rockies, who considered Garland during the offseason, to sign him.
The Rockies eyed Garland because of his career 1.31 groundball-to-flyball rate. Rockies scouts had Garland's rate this spring at around 1.7.
"He profiles for us and the things we're trying to do," Rockies manager Walt Weiss said. "He's a veteran and he's got some stripes on his shoulder from his experiences in the game. There is some youth to our club, so he helps us from a stability standpoint. We'll see how it goes."
Weiss earlier in camp announced four starters -- Opening Day starter Jhoulys Chacin, home opener starter Jeff Francis, Jorge De La Rosa and Juan Nicasio.
Lefty Drew Pomeranz was in line for a rotation spot. A 2012 rookie season that saw him go 2-9 with a 4.93 ERA in 22 Major League starts prompted him to spend the offseason working to recapture the pitching motion that led the Indians to select him fifth overall in 2010. He was traded to the Rockies in the Ubaldo Jimenez deal in 2011 and was in the Majors by season's end. Last year, he was trying to pitch a Major League-length season with little Minor League experience -- something rarely asked of a starter.
Early indications were that the Rockies would look at Pomeranz in a middle-relief role. However, Weiss said that is not necessarily the case. He had not spoken to Pomeranz as of Sunday, but said as it stands now if he isn't in the rotation he would more likely begin the year in Triple-A Colorado Springs.
Garland is 132-119 with a 4.32 ERA in 353 games (330 starts) for the White Sox, Angels, D-backs, Padres and Dodgers.
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.