It's been a long and arduous journey to Eugene for the San Jose, Calif. native.
Mattison began his career as an outfielder with the Marlins' after spending two years playing the same position at Virginia Commonwealth University.
He played 52 games in 2003 for the Marlins Single-A affiliate in Jamestown, where he hit .211 with one homerun and nine RBIs.
The organization told Mattison that they were not impressed with his power or his speed, but they did like his arm and decided to convert him to pitcher during spring training in 2004.
He'd pitched only six innings at VCU, and did not even touch the mound until he was used sparingly in high school out of necessity.
He started 2004 in rookie ball and pitched 11 games before he deemed his situation hopeless and decided to call it quits.
"I wasn't really happy being down where I was in rookie ball," Mattison admitted, posting a 6.00 ERA in 12 innings pitched. "I took it for granted."
Mattison spent the year off in Virginia and began a small career in financial services.
But baseball has always been a huge part of Mattison's identity, and he knew he must return to game he'd played nearly his entire life.
That chance came during an Independent League tryout, where Mattison performed well and caught the attention of Mal Fichman and Bill Bryk, two members of the Padres organization.
"They said, ‘we want to give you the opportunity,' and I said thank the lord," Mattison said.
Now with the Emeralds, Mattison is taking pride in coming to the ballpark each day.
"I find myself in a better place here," Mattison revealed. "I can't be happier … I come to the place I love every day. This is heaven on earth to me."
Mattison is currently second on the Emeralds with five saves and carries a 2.55 ERA in 17.2 innings pitched. He's been especially tough at home in Civic Stadium where he has not allowed a single run and has given up only three hits in 7.2 innings pitched.
"He's done a heck-of-a job for us," Eugene pitching coach Wally Whitehurst said. "He's come into some tough situations especially in save situations, and he's done a good job.
"The one thing we are trying to get him to do is be more consistent in the strike zone. He's done that better as of late. His last three or four outings, he's done really well. He's got a pretty good fastball. His slider is OK at times, and his changeup's OK. It's just getting him to learn how and when to throw those pitches, but he's done a good job for us."
Mattison admits he tries to keep an open mind when he enters the game.
"I'm pretty blank minded. I don't really think about too much, especially coming into a clean inning. So I just try to throw strikes."
In fact, Mattison keeps an open mind about nearly every aspect of his career. When asked about his strengths as a pitcher and repertoire, Mattison said he didn't really know.
"I don't understand my success so far," Mattison admitted. "I'm just glad they just keep hitting it to the good defense I've got behind me."
He also remains unconcerned by his statistics. He was unaware of his ERA difference at home (0.00) and on the road (4.50) and tries not to make it a habit to look too far into any statistical categories.
"Last time I looked at my stats, I had a zero ERA with the Marlins. I said ‘I don't think I've given up a run.' I looked at my stats and I hadn't. The next time out, I gave up four," Mattison said. "So, I stay away from that. I know I have a good defense behind me, so I just try to make them hit the ball."
So rather than statistics, Mattison said, he is motivated by his love for the game.
"The passion is what really drives me. Just being out here, smelling the grass. It's very stereotypical, but call me the next Jim Morris story. It's really what I'd love to be, but I guess I'm not as old as he was," Mattison joked.
While he may not be making a comeback at the same age Morris did, age is still a concern for Mattison. At 25, he is the oldest member of the Emeralds' roster, meaning the learning curve is much smaller for him.
"He is the oldest guy here. The window of opportunity for him is probably slight. But if you're still playing the game, there's always a chance. And I think he does have a chance if he continues to go out and continues to improve," Whitehurst said. "The biggest thing is to have his pitches be in the strike zone, be aggressive, and anything can happen. But he's got to continue to do that, and he's got to do that a little bit quicker than most guys here just because of his age. I think he understands that."
Mattison said is it odd being the oldest member on staff, but because he has been pitching less than most other members on the staff, he also feels like the rookie.
"No one comes to me. I go to everybody else," Mattison said. "I'm the rookie on the staff. Everybody's been pitching their whole lives. So I come to them …‘how do you throw your changeup, how do you throw your slider, what would you do in this situation?' I go to other people."
Though Mattison understands that making it to The Show is a distant hope, right now he's content being back in the game of baseball, and he hopes to make an impression on the fans and players in his first season with the Padres' organization.
So, when all is said and done, how does Justin Mattison hope his legacy will be remembered?
"That's a hard question," Mattison said. "I guess as someone who appreciates the game, and loves the game, took it for granted once, and will never do it again."