The 32-year-old right-hander asked for his release after refusing an assignment to Triple-A Tacoma. The club made the move Monday.
Baker was 1-1 with a 6.75 ERA in four spring training starts for Seattle, allowing nine earned runs and 16 hits in 12 innings.
Baker was in contention for a spot in the starting rotation, currently missing injured pitchers Hisashi Iwakuma and Taijuan Walker. Neither is expected to be ready for his first turn.
With ace Felix Hernandez the only certainty, the Mariners have right-handers Blake Beavan and Erasmo Ramirez and left-handers James Paxton, Roenis Elias and Randy Wolf in the mix for four available spots.
Baker spent his first six major league seasons with Minnesota and pitched briefly for the Chicago Cubs last year.