The adult male lion, described as emaciated and showing other signs of starvation, was later killed by wildlife officials after it attacked a dog brought in to track it. 
Dustin Britton, a 32-year-old mechanic and ex-Marine from Windsor, Colo., said he was alone cutting firewood about 100 feet from his campsite in the Shoshone National Forest when he saw the 100-pound lion staring at him from some bushes.
The 6-foot, 170-pound Britton said he raised his 18-inch chain saw and met the lion head-on as it pounced—a collision he described as feeling like a grown man running directly into him.
"It batted me three or four times with its front paws, and as quick as I hit it with that saw, it just turned away," he said in a telephone interview with The Associated Press.
Wildlife officials said Sunday evening's attack about 27 miles west of Cody was highly unusual because mountain lions are reclusive by nature. Only eight cases of mountain lions acting aggressively toward humans have been documented in Wyoming over the last decade.
Given the animal's low body weight—male lions in Wyoming typically weigh 140 to 150 pounds—it may have attacked Britton out of desperation, said Mark Bruscino, a Wyoming Fish and Game biologist who responded to the incident.
"Starvation is a strong motivator for any animal in chasing a meal," he said.
It was uncertain why it was starving. Britton's campsite, near the North Fork of the Shoshone River, was in an area heavily populated by the lion's typical prey—elk, deer and bighorn sheep.
Officials suspect disease may have played a role in the attack.
The wounded animal retreated after Britton inflicted a six- to eight-inch gash on the lion's shoulder, leaving him with only a small puncture wound on his forearm. 
"You would think if you hit an animal with a chain saw it would dig right in," he said. "I might as well have hit it with a hockey stick."
After the confrontation, Britton and his wife, Kirsta, decided to spend the night in their pop-up camper with their two children rather than risk packing up with the lion still on the loose.
Wildlife agents were called the next morning, after Britton told a passing U.S. Forest Service employee about the incident.
The family later carried on with their vacation, continuing to camp at sites in Wyoming and Montana.
Initial tests conducted on the lion for rabies and other diseases came up negative. Its carcass was taken to the Wyoming State Veterinary Laboratory in Laramie for further testing.
Bruscino, the state biologist, said mountain lions are known to inhabit the area where the attack occurred. They occasionally pass near campgrounds and other inhabited areas, but usually flee at the first sign of humans.
"It was acting completely out of character for how a lion would normally act around a full grown man who has a chain saw in his hand," he said.