The 25-year-old left-hander has never pitched above Double-A after a fine spring in which he had a 2.04 ERA in five games two starts. He walked eight and struck out eight.
Lefty reliever Joe Beimel also made the club, with the Mariners optioning pitchers Blake Beavan and Brandon Maurer to Triple-A Tacoma along with reliever Lucas Luetge.
Elias and Beimel were added to the 40-man roster.
Elias and recently signed right-hander Chris Young will take the spots of injured starters Hisashi Iwakuma and Taijuan Walker, who are expected to begin the season on the disabled list.
Felix Hernandez pitches the opener followed by Erasmo Ramirez on Tuesday, lefty James Paxton on Wednesday, Elias on Thursday at Oakland and Chris Young Friday against the Athletics.
Also, the Mariners optioned infielder Nick Franklin to Tacoma, clearing the way for Brad Miller to open the regular season as the team's starting shortstop after a strong spring training in which he batted .439 in 19 games.
The Mariners re-assigned pitchers Zach Miner, Dominic Leone, Ramon Ramirez and Carson Smith to minor league camp. Catcher Humberto Quintero and outfielder Endy Chavez joined them, and outfielder Xavier Avery and infielder Carlos Triunfel were designated for assignment.
Seattle opens the regular season Monday in Anaheim against the Angels.
Click here for a look at the Mariners 25-man roster.