And while he's been used mostly in low-leverage situations so far, the former Orioles starter is 1-0 with a 0.90 ERA, having allowed five hits with seven walks and 13 strikeouts in 10 innings.
Johnson struck out the last two batters he faced on Wednesday against the A's, then whiffed five straight Twins in a two-inning stint on Friday. Charlie Furbush is the only Mariners reliever to have recorded more than seven straight outs by strikeout since 2010, having racked up nine in a row in 2013.
So how exactly is the 6-foot-1, 220-pound right-hander fooling hitters?
"It's a deceptive delivery," manager Scott Servais said. "He's got the ball up in the zone. We call it the invisa-ball, the 88-89 mph fastball that they swing through. Hitters are so accustomed to seeing the ball come in at a certain angle and that's where they know it's going to be and that's where they swing, but the ball's not there.
"For whatever reason, he's got a little more spin rate and the ball stays up in the zone. And even though it's not 95, it plays up like it is a higher velocity fastball. And good for him. He's thrown the ball well in the spots we've put him in."
Johnson went 4-0 with a 2.11 ERA in 12 games, including four starts, for the Orioles in 2012, but spent most of the past three seasons in the Minors. He sports a 3.76 ERA with 82 strikeouts in 69 1/3 innings in his MLB career.
Johnson says he doesn't try to overanalyze why his medium-paced fastball seems to give many hitters trouble.
"I'm kind of able to get away with that higher pitch," he said. "I know that's a pitch they normally like to swing at, so if I can get it in that area - obviously you want to keep things away or go in with it -- but I know when I'm ahead if I go there or anywhere in the zone where they want to swing, it can lead to a swing and miss or a pop up or something like that."
Servais said he's not afraid to pitch Johnson in any situation and his role could expand if he continues performing well. For Johnson, it's just a matter of taking advantage of whatever chances come his way.
"Anytime I can get in there and try and prove what I can do more and more, hopefully it will get them to gain more confidence in me and put me in some higher situations if they need," said Johnson. "That's the goal. They're kind of getting to know me as well, since they didn't get to see me in spring. I'm just trying to get as many opportunities as possible and show them what I can do in each one."