Emmons, a 23-year-old graduate student in accounting, scored 703.3 points while Lusch took the silver medal with 702.2. The German had stalked the American, moving to within just 0.3 point of Emmons with two rounds left.
But Emmons, a deer hunter from New Jersey, stopped Lusch with two bullseye scores of 10.6 on his final two shots.
Emmons, who was taught how to shoot by an FBI firearms instructor at the age of 14 and later earned a partial scholarship at the University of Alaska, held a one-shot lead from the qualifying round over Lusch, who is also 23.
CONSECUTIVE BULLSEYES
Cheered on by a large crowd of German backers at the Markopoulo shooting range outside Athens, Lusch had gradually closed in on Emmons, shooting nine consecutive bullseyes.
With the pressure building, Emmons recovered after a weak 9.9 in round six and nailed his final two shots with a pair of 10.6s. Lusch, meanwhile, slipped to a 9.9 on his final attempt, finishing second.
The two young outsiders nevertheless both leapt up off the mat and celebrated their medals over an experienced field that included all three medal winners from the 2000 Olympics.
"I wasn't worried about what everyone else was doing but I knew it was close in the middle rounds," said Emmons. "I just stayed focused on what I was shooting. I was pretty relaxed."
Lusch said he was delighted with the silver medal.
"I knew he was still just ahead but I thought I could catch him," said Lusch. "It was looking good until the final round. There was an awful lot of pressure. I'm very happy with silver."
Sergei Martynov of Belarus won the bronze medal, repeating his performance at Sydney. He moved up from fifth at the start of the final 10-shot round to overtake Slovakia's Jozef Gonci who finished fourth.
Defending Olympic champion Jonas Edman of Sweden failed to qualify for the final, finishing a distant 32nd in the 46-man field with 590. The 2000 silver medallist, Torben Grimmel of Denmark, also fell short of the finals with 594 and ninth place.
Emmons, who trained full time since finishing school in December and plans to start graduate school in Colorado after the Olympics, said he was not fazed by the experienced field.
"I wasn't bothered by that at all," he said. "I knew if my scores were good enough no one would have a leg up on me."