Sheffield signed a minor-league contract with Atlanta on May 4.
Sheffield has been assigned to the organization's Florida Complex League team and its season hasn't started yet, so Atlanta might be having him work on his mechanics before elevating him to a full-season affiliate. The left-hander, a first-round pick back in 2014, was released by the Mariners last month after allowing 13 earned runs across his first 8.1 innings with Triple-A Tacoma this year.