Leone has allowed just one run in 7 1/3 innings over seven Cactus League appearances, giving up four hits with two walks and four strikeouts.
And despite his lack of physical stature, the Connecticut native fires fastballs in the mid- to upper-90s, which tends to open eyes when hurled with accuracy.
What exactly has new skipper Lloyd McClendon seen in the youngster who was just elevated to Double-A Jackson in midseason last year?
"Ninety-seven miles per hour," McClendon said. "That's a lot to like. He's got some quality pitches and great secondary stuff. He's definitely in the mix. We'll see how it all plays out."
The Mariners have had a run of young power arms arrive the past few years, with mixed results. Stephen Pryor was impressive in his jump to the Majors, but then hurt his shoulder and is just coming back. Carter Capps approached triple digits on the radar gun, but had control issues and was traded to the Marlins for Logan Morrison.
Tom Wilhelmsen sped to the Majors and took over as closer for a full year, but then lost that job and is working to regain his mojo this spring. Danny Farquhar was acquired in a trade from the Yankees and has been very effective in his short stint in the big leagues.
Carson Smith, a 6-foot-6 flame thrower drafted in 2011, started drawing attention at camp last year and is again throwing well this spring after saving 15 games at Jackson last season.
But there hadn't been a lot of talk about Leone before now, given he started last season in Class A Clinton, then moved to Class A Advanced High Desert before bumping up again to Jackson on July 13.
He started out this spring as one of the many bodies crammed in the far end of the Mariners clubhouse where all the Minor League non-roster invitees hung out.
But as the herd thinned over the past two weeks, Leone remained standing like a lone sentry in the long-shot corner until finally, he and fellow survivor Roenis Elias discovered their lockers had been moved down with the veteran group.
"It was a little lonely down there," Leone said with a smile. "But I'm not going to complain. At least I'm still here."
Leone, who played college ball with shortstop Brad Miller, clearly has made the most of his first Major League camp. Mariners officials have been quietly keeping his name alive in conversations about potential bullpen options as the spring has progressed.
"It's a lot of fun just to come out here and try to show what I can do for everybody who might not have seen me before," Leone said. "Especially with McClendon coming in with a new staff, it kind of gives me more incentive to really come out and push the pace.
"When I got the invite, I took it as this is my time to really open some eyes and show people what I can do. And so far this spring, I think I've done that. I've also learned a lot of stuff, too, which I can use to improve my game."
Leone was primarily a starter at Clemson, where he went 16-8 with a 4.63 ERA over three years. But when he arrived at Class A Everett in 2012, he was assigned to the bullpen to finish out that season, and the switch stuck.
"They didn't want to overtax me, being a college guy coming in," Leone recalls. "So they said, 'go to the bullpen', and it worked out. I was throwing well and I know that fall they had conversations about whether I'd start or relieve, but I came in last spring and they never really said anything, they just kept me in that bullpen role. So I just ran with it."
He ran with it well enough to rack up 16 saves in 17 opportunities, not allowing a run and holding opposing hitters to a .095 average with 28 strikeouts and just three walks in those save situations. Leone then led the Arizona Fall League with six saves for the Peoria Javelinas, and has carried that success over to this spring.
The Mariners certainly don't need him in closing situations with Fernando Rodney signed on for those duties, and former closers Wilhelmsen and Farquhar also in the 'pen. But Leone could slot into a middle-inning situation if he makes the team, with a chance to gather experience and soak up more from his veteran teammates.
"I watch Rodney a lot," Leone said. "I watch a lot of the guys in the bullpen because they've all been at that highest level, so why not pick up what you can? It's a full view in front of you, why not take bits and pieces from everybody?"
His biggest lessons so far?
"Everyone has their own way of going about their business in the 'pen and in workouts or whatever, but when they get on the mound, guys are different people," he said. "I feel like I'm the same way. I can be relaxed and go through my work here, but when I get on the mound I'm completely focused, so I like to look at those guys and how they focus in on the mound, how they attack, what's their game plan?
"Because mechanical stuff, everyone is different. Rodney doesn't have a big leg kick, but he still generates incredible power. Then you've got guys like Pryor, who has a big leg kick for a big guy. So I just try to look at the mental side of it and try to figure out how to be a reliever mentally because physically, you have to have the tools to do it."
Leone was blessed with a pretty valuable weapon of his own, an ability to generate considerable power from what he jokingly refers to as his "lower center of gravity." He also has a capable cutter and slider along with a developing changeup to keep hitters off balance.
But certainly it is the heater that has caught eyes. How exactly does a 5-11 guy generate such power?
"I really kind of simplified my mechanics and kept everything on line, rather than a lot of moving parts," Leone said. "And just staying in the weight room, staying healthy, staying consistent with what I do just allows me to be really efficient with how I throw the ball. That's about as crazy as it gets. I just go out and throw.
"But I also know that fastball command and location is the ultimate key. If you can throw hard, but you can't throw strikes, it doesn't do you much good. You've got to be able to put it where you want, and luckily I've been doing that so far this spring and hope to continue that into the actual season."