"I just did what I was trained to do," he says in a tone that is neither defensive nor boastful. "I was in-country a long time in a very hot area. I didn't do anything special."
By all accounts other than his own, Mawhinney is a master of one of the military's most dangerous, deadly and misunderstood roles.
In 16 months as a Marine Corps sniper in Vietnam in 1968 and 1969, he killed 103 of the enemy. An additional 216 kills were listed only as probables because it was too risky to take time to search the bodies.
No other Marine sniper in Vietnam had more confirmed kills of Viet Cong and North Vietnamese Army regulars than Mawhinney.
Yet for more than two decades after he left the Marines in 1970, only a few fellow Marines knew of his assignment. At home in Oregon, he never told even his closest friends.
But a tell-all paperback by a friend and fellow Marine sniper - "Dear Mom: A Sniper's Vietnam," by Joseph Ward - flushed him out.
Even in an age of million-dollar, computer-driven missiles, the ability of one man to kill another with a 20-cent bullet is a much-prized skill among military forces.
Mawhinney is now in heavy demand within military circles to describe his techniques, his emotions and his assessment of what he accomplished from ambush.
At first embarrassed and annoyed at losing his privacy, Mawhinney reluctantly decided to tell a cold tale of killing in service to country.
"Once I had a Charlie (slang for Viet Cong) in my scope, it was my job to kill him before he killed me," said Mawhinney, 51 and retired from a desk job with the U.S. Forest Service. "I never looked in their eyes, I never stopped to think about whether the guy had a wife or kids."
He was routinely deadly from 300 to 800 yards - and had confirmed kills at more than 1,000 yards.
"It was the ultimate hunting trip: a man hunting another man who was hunting me," he said. "Don't talk to me about hunting lions or elephants; they don't fight back with rifles and scopes. I just loved it. I ate it up."
He would much rather be talking sports or deer hunting with friends. But for two years in a row he has been the top speaker at an international symposium on sniping, near Washington, D.C.
So what changed his mind about never rehashing Vietnam?
First, anonymity no longer was an option, so he decided he could help change the public image of snipers as bloodthirsty assassins. A good sniper, Mawhinney said, saves more lives than he takes because he undercuts the enemy's will or ability to fight.
On the wall of the Marine sniper school at Camp Pendleton is a Chinese proverb: "Kill one man, terrorize a thousand."
Second, going public offered a chance to say something that might help some other scared serviceman stay alive someday.
Most combat is fought with automatic or semi-automatic weapons. Snipers generally fire one shot at a time from bolt-action weapons that provide greater accuracy and distance but leave them virtually defenseless against automatic weapons at close range.
In Vietnam, the enemy put a bounty on the head of U.S. snipers. Mawhinney carried a sidearm with a round to fire into his temple rather than be captured.
He shipped out to Vietnam during heavy fighting that followed the Tet Offensive in early 1968. As a sniper, Mawhinney had an uncanny ability to gauge distance, moisture, weather and terrain - factors that determine how much a bullet will rise or drop during flight. He had the patience to wait hours for the right shot. He was scared but exhilarated.
"Normally I would shoot and run, but if I had them at a (long) distance, I wasn't worried," Mawhinney said. "I would shoot and then lay there and wait and wait and wait, and pretty soon somebody else would start moving toward the body. Then I would shoot again.
"When you fire, your senses start going into overtime: eyes, ears, smell, everything," he said. "Your vision widens out so you see everything, and you can smell things like you can't at other times. My rules of engagement were simple: If they had a weapon, they were going down. Except for an NVA paymaster I hit at 900 yards, everyone I killed had a weapon."
Near the An Hoa base outside Da Nang, he caught a platoon of North Vietnamese army regulars crossing a stream. He hit 16 with head shots with an M-14, which he often carried in addition to his bolt-action.
The 16 were listed only as probable kills because no officer was there to see their lifeless bodies float by.
He retains an intimate knowledge of what people look like in the throes of death.
"Sometimes, depending on where they're hit, they'll just drop and not move," Mawhinney said. "Nobody dies the same, and I've seen it all. I did a lot of mercy-shooting. I wounded people and then cranked another round into them. I didn't want them crawling around out there."
He eventually became disillusioned with American objectives in Vietnam. Still, he extended his tour of duty twice to help keep fellow Marines alive.
After 16 months as a sniper, a chaplain thought he was suffering combat fatigue. His days of killing were over.
Assigned as a rifle instructor at Camp Pendleton, he had nightmares of being back in Vietnam, trapped in a foxhole and unable to return fire with his bolt-action as enemy rounds poured in. "I could feel the bullets hitting me," Mawhinney said.
He left the Corps and returned to rural Oregon. Within three days of returning, he had a job with the Forest Service. After a while, the nightmares went away.
"I felt I was finally home, not like when I would come home on leave from Vietnam and knew I had to go back to that hell," he said. "I'm not a guy who looks back. Vietnam was something I had to do in that part of my life. I try to do everything 100 percent. If you're a sniper, that's the only way to do it, if you want to stay alive."