Mr. Triplett would make medical history as “Case 1,” the first person formally diagnosed with autism. His upbringing and behavior were described at length in a 1943 scientific article by Austrian American psychiatrist Leo Kanner, “Autistic Disturbances of Affective Contact,” which outlined the developmental disability now known as autism spectrum disorder, or ASD.

The article went on to describe 10 other autistic children, most of whom were locked away in state schools and hospitals while experiencing communication and behavior challenges. Checking in with his former subjects almost 30 years later, Kanner would write that institutionalization was “tantamount to a life sentence … a total retreat to near-nothingness.”

Mr. Triplett, by contrast, gained acceptance and admiration while remaining a part of his community. With support from his family, which could afford to send him to Kanner and which later set up a trust fund to look after him, he graduated from college, got a job as a bank teller and found companionship in a morning coffee club at City Hall. He played golf, sang in a choir and traveled the world, visiting at least three-dozen countries and making it to Hawaii 17 times.

By choice, he traveled alone, surprising relatives when he would announce at Sunday dinner that he had recently returned from seeing a golf tournament in California or, in search of an oyster dinner, driven his Cadillac to New Orleans.

Mr. Triplett, who was known as D.T. or just plain Don, was 89 when he died June 15 at his home in Forest, Miss., where he lived virtually his entire life. The cause was cancer, said his nephew, O.B. Triplett.

“Donald was given the opportunity to pursue his passions and his interests, and he was able to build a very happy life for himself on his own terms,” said Christopher Banks, the president and chief executive of the Autism Society, an education and advocacy group. “He was known in his community, he was accepted in his community, and he was celebrated in his community. All of that demonstrates the importance of building an inclusive society.”

Mr. Triplett’s place in history was largely forgotten until journalists John Donvan and Caren Zucker tracked him down while researching the history of autism, and interviewed Mr. Triplett for an 8,000-word article that ran in the Atlantic in 2010. They later expanded their reporting into a 2016 book, “In a Different Key,” which became a finalist for the Pulitzer Prize and was adapted into a documentary that aired last year on PBS.

“By the time we first met Donald, his generational peers and those who followed had all experienced a lifetime of knowing that Donald was different without thinking that it was a big deal,” Donvan and Zucker wrote in an Atlantic article after his death.

“He was blessed with friends,” they added, “whose explanation for their fondness or love for Donald was always, simply, that he was a great guy. The entire town, in fact, was unaware that he had an autism diagnosis until we first brought that news in 2007. After that, the community became extra proud of the minor celebrity in its midst.”

When Mr. Triplett was diagnosed with autism as a child, the condition was considered extremely rare. Since then, researchers have found that about 1 in 36 children are autistic, according to estimates from the Centers for Disease Control and Prevention. No medical test exists to identify the wide-ranging disability, which can vary in form and severity and is diagnosed by monitoring a child’s development and behavior.

Researchers are still debating autism’s cause. But “to a remarkable degree,” Donvan and Zucker wrote in their original article, the broad outlines of autism remain largely the same as what Kanner traced in his study of Mr. Triplett, who was 5 when he accompanied his parents on a train ride north to Baltimore to meet the child psychiatrist who later referred to him anonymously as “Donald T.”

The older of two sons, Donald Gray Triplett was born in Forest on Sept. 8, 1933. His father, Beaman, was a Yale-educated lawyer whose own father had served as the town’s mayor. His mother, the former Mary McCravey, was also part of the town’s elite, from a family that helped found the Bank of Forest.

By age 2, Donald was reciting the 23rd Psalm (“The Lord is my shepherd”) from memory. He had little interest in eating food and seldom paid attention to the people around him, even when his parents arranged a meeting with a man dressed as Santa Claus.

A few weeks before he turned 4, his parents sent him to a state-run institution 50 miles from home, in a Mississippi town called Sanatorium. Donald turned inward, sitting motionless and paying little attention to music or toys, until his parents took him out after about a year, against the advice of a facility director who claimed that Donald was “getting along nicely.”

In October 1938, they took him to see Kanner, a professor at Johns Hopkins University. The psychiatrist kept Donald under observation for two weeks and later stayed in touch with the family, which brought Donald for three follow-up visits as Kanner began identifying other children with similar symptoms, preparing what became his first article on autism.

Mr. Triplett went on to graduate from high school and study French at Millsaps College in Jackson, Miss., where he received a bachelor’s degree in 1958. He joined the Bank of Forest later that year and continued working there off and on for nearly 65 years, living with his parents and remaining in their home after they died in the 1980s. His brother, Oliver, helped look after Mr. Triplett until his death in 2020.

Mr. Triplett has no immediate survivors but leaves a large community of friends. He had assigned nicknames and even numbers to many of them, in a kind of personal inventory system that led him to refer to one fraternity brother as 569 and another acquaintance as 333.

When outsiders came calling, his friends and neighbors turned protective, as Donvan and Zucker discovered when they first arrived in Forest. “On three occasions,” they wrote, “while talking with townspeople who know Donald, we were advised, in strikingly similar language each time: ‘If what you’re doing hurts Don, I know where to find you.’ We took the point: in Forest, Donald is ‘one of us.’”