A man from upstate New York was found guilty on Tuesday of second-degree murder for fatally shooting a young woman who was riding in a car that had mistakenly driven up his driveway. He had argued that he fired the fatal shot by accident.

On the night of April 15, the defendant, Kevin Monahan, fired two shots with a .20-gauge shotgun, one of which struck a car carrying Kaylin Gillis, 20, who was hit in the neck and died soon after. Ms. Gillis and a group of six friends had been trying to find a friend’s house for a Saturday night party when they drove up Mr. Monahan’s half-mile-long driveway in the rural town of Hebron, N.Y., about 55 miles north of Albany.

During the two-week trial, Mr. Monahan, 66, had argued that he had not meant to shoot at the car carrying Ms. Gillis, but had tripped on his porch. He said he had initially fired a warning shot after seeing a caravan of two cars and a motorcycle arrive at his house at night.

He was described by his lawyers as “an old man” who had been awakened in bed and had been terrified that “a group of marauders” had come to attack him and his wife, Jinx, who was hiding inside the house, also armed with a gun.

But the authorities had expressed doubts about the level of danger Mr. Monahan might have felt that night, noting that the vehicles were turning around to leave. And prosecutors had expressed deep skepticism about his defense argument, saying in a closing statement on Tuesday that Mr. Monahan had acted with animus and callous disregard for the people who had stumbled onto his property.

“Kevin Monahan did not act out of fear,” said Christian P. Morris, the first assistant district attorney for Washington County, speaking to the jury before a nearly full courtroom in Fort Edward, N.Y. “He acted out of a baser emotion than that: He acted out of anger.”

The jury also seemed to reject Mr. Monahan’s explanation; it took less than two hours to return its three guilty verdicts, including reckless endangerment and tampering with evidence, related, in part, to his efforts to clean the shotgun after the shooting.

Mr. Morris said Mr. Monahan had shot twice in rapid succession, even though the cars and motorcycle were in the driveway for about 90 seconds.

The vehicles were “interrupting his night,” Mr. Morris said, so Mr. Monahan “grabbed his shotgun and intended to make them leave as fast as possible. And he didn’t care if they were hurt or killed.”

Prosecutors also used body-cam footage and 911 calls to show that Mr. Monahan and his wife had initially lied to authorities, telling a police officer who arrived shortly after the shooting that they had had no visitors that night. Mr. Monahan also feigned confusion about why neighbors had heard gunshots, Mr. Morris said, suggesting that hunters were prowling the woods behind his house, after dark.

“It’s a total farce,” he said.

Ms. Gillis’s death stunned the local community and reverberated around the nation, another random killing in a country all too accustomed to gun deaths. Days before the shooting in New York, there had been another shooting in Kansas City, Mo., of a Black teenager who had approached the wrong house while trying to pick up his brothers.

Ms. Gillis’s friends had testified to a terrifying scene the night of the shooting, with the sudden flash of the shotgun and impact in the car carrying Ms. Gillis. She was hit in the spine.

In the moments following the shooting, the victim’s boyfriend frantically tried to find a cellphone signal — often scarce in rural New York — to call 911 as others tried to perform CPR.

Mr. Monahan had a reputation around town as a somewhat surly character and had posted “private property” signs, warning off trespassers, and a small “private drive” sign at the base of his driveway.

But his defense attorneys insisted that the shotgun had fired on its own after Mr. Monahan — who was wearing flip-flops — stumbled on a nail and ran into a railing, an argument they said was supported by a single test of the weapon by a New York State Police investigator in which it discharged after being dropped. (The gun did not discharge during a series of other tests.)

Ms. Gillis, who had wanted to become a marine biologist, was mourned by friends and family, many of whom filled the courtroom during the trial. That included her father, Andrew, a correction officer, who said he wanted Mr. Monahan to be harshly punished.

“I just hope to God he dies in jail,” Mr. Gillis said shortly after his daughter’s death.

The judge, Adam D. Michelini, will issue his sentence on March 1.

Mr. Morris also noted that Mr. Monahan was standing on an elevated porch, more than 20 feet above the vehicles — “He had the high ground,” the prosecutor said — which were about 80 feet away and turning around.

“There was no threat; there were no marauding raiders,” Mr. Morris. “These were lost kids.”