Millwood made nine starts with the Colorado Rockies late last season after spending time in the minor league organizations of the New York Yankees and Boston Red Sox. He was 4-3 with a 3.98 earned run average for Colorado after joining the Rockies in early August.
The 37-year-old right-hander owns a career record of 163-140 with a 4.10 ERA in 423 games, all but eight of which have come as a starter. He broke in with Atlanta in 1997 and pitched for the Braves through 2002, then spent time with Philadelphia, Cleveland, Texas and Baltimore over the following eight seasons.
"Kevin brings a great deal of experience as a veteran pitcher and will compete for a spot in our starting rotation," said Mariners general manager Jack Zduriencik in a statement. "His leadership and experience will be a benefit for our young pitchers and we look forward to seeing him in spring training."
The Mariners lost one of their top pitchers from last season earlier this week when they dealt Michael Pineda to the New York Yankees for top catching prospect Jesus Montero.
Millwood appeared in the postseason with Atlanta from 1999-2002, and was an All-Star with the Braves in 1999.