"I was just looking for something I could get up and elevate to the middle of the field," said Zunino, last year's first-round Draft pick. "He usually throws a bunch of sliders, so I was looking for one of those early. But he kept coming with the fastball, so with a 3-2 count I knew I had to stay aggressive. With the infield playing in, I knew I got enough of it to get it through."
Zunino wasn't the only youngster contributing. Rookie second baseman Nick Franklin went 3-for-5 with two doubles and a run scored to lift his batting average to .302 after his first month in the Majors.
And shortstop Brad Miller made his Major League debut a memorable one as he drove in a run, stole a base and made several key defensive plays before he stood in the on-deck circle and watched Zunino drive in the game-winner.
"A walk-off win in my first game, that was awesome," said Miller, who was called up from Triple-A Tacoma earlier in the day. "I mean, shoot, Zunino battling like crazy and sending one right up the middle, that was pretty sweet. I'm still on cloud nine. That was awesome."
Cubs reliever Blake Parker walked Michael Saunders and Raul Ibanez to lead off the bottom of the 10th before Jason Bay sacrificed both runners into scoring position. The Cubs then intentionally walked Justin Smoak to set up a bases-loaded situation for Zunino against Camp.
Mariners right-handed reliever Yoervis Medina picked up the victory after shutting down the Cubs in the top of the 10th.
The win snapped a two-game losing streak for Seattle.
An RBI double by Kendrys Morales and run-scoring triple by Ibanez pulled the Mariners into a 4-4 tie in the bottom of the eighth and took Hisashi Iwakuma off the hook from suffering his third straight loss.
Iwakuma has been one of baseball's premier pitchers for much of the first half, but the Mariners right-hander gave up four runs for a third straight game and was in line for the defeat until Seattle's late rally.
Iwakuma, 32, wound up with a no-decision and still figures to be in strong contention for an American League All-Star selection at 7-3 with a 2.42 ERA. But it's not quite the slam dunk things appeared when he sat 7-1 with a 1.79 ERA before losing twice to the A's and now a no-decision to the Cubs.
Iwakuma has one more start on Thursday in Texas before All-Star selections are announced on Saturday, July 6.
Iwakuma gave up four runs on six hits with no walks and five strikeouts in eight innings. He's allowed four runs in each of his last three starts after surrendering more than three runs in a game just once in his first 14.
"Overall, I thought I pitched well today," Iwakuma said through translator Antony Suzuki. "I was able to get strike one for the most part and get ahead early in the count. Besides the two home runs I gave up, they were to the fat part of the plate. Otherwise, I felt good."
The Cubs tallied two runs in the second on an RBI triple by Ryan Sweeney into the right-field corner, with Sweeney then scoring on a throwing error by Franklin on the relay throw home as the ball got away from Iwakuma and rolled into the dugout as he tried backing up Zunino.
Iwakuma gave up only one hit in the next four frames before solo home runs by Alfonso Soriano and Dioner Navarro in the seventh made it 4-1. Iwakuma has given up five home runs in his last two starts after allowing just 10 in his first 15 games.
The Mariners got one run of their own in the fifth on a leadoff home run by Bay. It was Bay's ninth long ball of the season and all nine have been solo shots.
Seattle managed only two other hits off Cubs left-hander Travis Wood in his 6 1/3 innings, a leadoff double by Franklin in the fourth and a base hit by Morales in the sixth.
Miller cleanly fielded a ground ball on the first play of the game, then made several tough over-the-shoulder catches as well as a diving stop of a hard-hit shot up the middle by Starlin Castro in the eighth.
"I thought he was outstanding," said manager Eric Wedge of Miller. "I thought he had a solid first game. First ball came to him, that's always a plus, get that out of the way. The play going to foul territory, he went a long way to get that ball. He made the dive, stole a base, got on base with a walk, went up there ready to hit. It was a solid first night."
Dustin Ackley got his first start in center field after being recalled from Tacoma on Tuesday and went 0-for-4.