NFL kicker Ryan Longwell was in third, and Dallas Cowboys quarterback Tony Romo and hockey Hall-of-Famer Brett Hull were tied for fourth at Edgewood Tahoe Golf Course.
Romo, who narrowly lost to Wagner last year, birdied the final hole to climb within striking distance.
"There are certain putts that you need to make to keep yourself in position," Romo said. "Even when you're not putting well, I had to have that one. It was important. I hit the ball fine, it was just some chips and some putts. If I clean that up, I'll have about 10 extra points."
Wagner — the defending champion, two-time winner and the only non-professional athlete to have won the tournament in its 23rd year — had three birdies to offset two bogeys for his total of 22 points in the modified Stableford scoring system.
"I think I had 40 putts. Normally, that's the strength of my game," said Wagner, who is one of four players who have played in every Tahoe event. "I hit it really well. I didn't make any putts."
Mulder, whose best finish in two previous tournaments was 14th in 2010, had four birdies to go with four bogeys.
"I was killing my drives and had mid-irons into the par 5s, but this is not what I do for a living so I'm not that confident in my game," Mulder said. "I'm trying to enjoy myself and not put so much pressure on myself this year. I came into this week hitting the ball well."
Longwell was one point behind the leaders and Romo and Hull had 20 points.
"Right now, as can you see by the leaderboard, nobody has come out of the pack," Wagner said. "Nobody's hot at all with the putter or probably playing that great."