Harper has never appeared in the Major Leagues and had to be added to the 40-man roster, which is now full again after being at 39 when infielder Mike Freeman was claimed off waivers by the Dodgers on Friday.
"From Day 1, it's been my goal to get here," said Harper, a 37th-round Draft pick of the Braves out of Austin Peay State University in 2011. "To actually do it, I don't know if it's really hit me yet. It's been a big blur the last two days. I had this goal in mind, no matter where I was I tried to do the same thing every day and keep the same mentality. I made it, and it feels good. All you can do is smile. It's like a smile is stuck on your face."
Harper was acquired by the Mariners from the Braves on Dec. 11, 2015, in a trade for right-hander Jose Ramirez. He made 42 relief appearances with Double-A Jackson last season, going 4-5 with six saves and a 2.51 ERA with 95 strikeouts in 68 innings.
Whalen made a spot start for Seattle on Saturday, allowing five runs in 5 1/3 innings in his Mariners debut. The 23-year-old was acquired from the Braves last winter.
The Mariners have already used 27 pitchers and 43 players overall in the first 50 games this season, the most in the Majors.
Harper wasn't in big league camp and never even pitched in a Cactus League game as a Minor League callup, so manager Scott Servais had never met the Tennessee native until Harper introduced himself on Sunday morning in the Mariners' clubhouse.
"I can honestly say I've never seen Ryne Harper pitch, but it won't be the first time this year," Servais said. "We've had a few of those. He's excited, first day in the big leagues. It's great to see a guy who is kind of overcome. He's a very late-round Draft choice, comes into the organization and gets 'em out. From what I understand, he's got a real good curveball and works off of that."
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB. This story was not subject to the approval of Major League Baseball or its clubs.