He was slotted in the fifth spot and went 0-for-4 with a pair of strikeouts in the Mariners' 5-4 win.
Pena hit 25 home runs in 76 games between Reno and Tacoma in the Pacific Coast League and five in 17 games with the D-backs in a brief stint in the Majors this season.
"I think this is my best year," Pena said. "I said to myself, like wine, when they're old, they taste better. I'm getting old, I think I've learned how to hit. It feels so good. This year, I'm hitting like I never have in my life. I was impressed with myself. I just have to do the same thing I was doing down there. Keep doing it here. Like I was helping the teams down there, do the same thing up here."
That would be a welcome scenario for the Mariners, but it should be noted that Pena has never quite panned out as expected since hitting 26 home runs for the Reds in 2004 and then .301 in 84 games with the Red Sox in '06.
He has a .251 average and 82 home runs over 577 career games with the Reds, Red Sox, Nationals and D-backs in parts of eight seasons, but was released by Arizona after batting just .196 in 46 at-bats earlier this season.
"He has tremendous power and some big league experience," Mariners manager Eric Wedge said. "We've got him in there tonight and will primarily use him in the DH role when he is in there. We still want to mix and match guys."
The right-handed Pena is one slugger who shouldn't be intimidated by the size of Safeco Field. He's not the type of hitter who has to worry about park dimensions.
"Yeah, I think that's fair to say," Wedge said with a smile. "He has a tremendous amount of power. Balls look different off his bat."
Pena reached well into the upper deck in left field during his initial batting practice. Now the key is translating that into games. And, no, he isn't worried about Safeco's tough reputation for right-handed hitters.
"I knew that was coming," he said, laughing, when asked about Safeco. "I know it's a big park. Everyone was saying, 'Oh, it's not a big park for you.' I'm not worried about a big park. I'm just worrying about being comfortable and doing the simple things to help the team.
"I'm not thinking I have to hit a home run. No. That will come. When I was down [in the Minor Leagues], they have some big parks. I just tried to make contact. If I make contact, something will happen."
Red Sox manager Terry Francona well remembers Pena's long-ball prowess from his two seasons in Boston, where he hit 16 home runs in 2006-07.
"It's silly," Francona said. "Wily Mo's power is off the charts. He hit a ball in Baltimore, it won the game for us. By the eighth inning it was a day when the wind was blowing and he hit it left-center. I couldn't imagine someone hitting a home run that day.
"He hit a homer in Kansas City that almost hit the concession stand. I mean, his power was just unbelievable. ... There were a lot of misses in there, too."
For his part, Pena just wants a chance to play and show what he can do to bolster a Seattle offense that ranks last in the Majors in scoring.
"It's an opportunity," he said. "I know they have great team. Some things aren't working the right way, but it's not going to be like that the whole year long. Hopefully it will change. It's going to start tonight. It's a different day, the past is the past. Now we have to start doing it from now on. I just want to help."