The bizarre moment came with no outs and two runners on base as Seattle starter Nathan Karns was battling to hold a 5-1 lead. The fan dropped over the eight-foot fence in left-center field and sprinted toward the infield right in front of Smith as he moved to his left to make the catch.
A security guard quickly tackled the fan, who was escorted off the field.
While Smith downplayed the incident, Karns didn't hold back on his thoughts.
"I think fans need to [stay] in the stands and let us play the game. I mean, he could have got someone hurt. It was in the middle of the play, fly ball. Who knows what state of mind he's in," Karns said.
"Seth Smith did a great job staying focused with the guy out in the field. But fans, they need to stay in the stands. This is our job. This is what we do. I don't know if he thought it was a smart idea, but he's probably going to get a hefty fine, he's probably going to spend the night in jail, and he's probably going to be banned from the stadium. So I really don't think it's a great idea. Once again the fans need to respect that their place is in the stands and ours is on the field."
Both managers were just thankful the play went uninterrupted.
"That's a shame it happened that way," said Cardinals skipper Mike Matheny. "If he interferes with a fielder, I don't know what they're going to do. It's going to be a mess. He was already committed [before the ball was hit]. I don't even know if he knew what state he was in. He had already jumped the fence and started his trek to fame and fortune before he even knew where that ball was going to be. I don't think it affected the play a whole lot."
Said Mariners manager Scott Servais: "Crazy play and in the middle of the action, you don't usually see it happen. It's like the fan timed it up or whatever. Seth said he started to freak out because he knows Leonys is way over there, but then you see somebody run in front of you like that. He kept his composure, caught the ball and got it in. It's just too bad. Fans should not do that, obviously."
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011.
Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.