Williams died Saturday at his Los Angeles home of complications from pancreatic cancer, his former publicist, Rob Wilcox, told the Associated Press.
He was the rare instrumental pop artist to strike a lasting commercial chord. Between 1955 and 1972, Williams had 22 hit singles — including "Born Free" in 1966 — and 38 hit albums, according to the All Music Internet database.
"Autumn Leaves" was the most successful song of Williams' career, and the first No.1 instrumental song on the Billboard charts of the rock era, according to "The Billboard Book of Number 1 Hits."
After Williams was signed to Kapp Records in the early 1950s, label president Dave Kapp asked Williams on a Friday if he would like to record "Autumn Leaves" the following Monday.
"I said, 'You mean 'Falling Leaves'? I didn't even know the title," Williams told The Times in 1996. "I stayed up Friday and then Saturday night working on an arrangement."
"The first thing that came to mind was to play all those runs down the keyboard," Williams said in an earlier Times interview. "I tried to make it sound like falling leaves."
The first recording run-through ran slightly over three minutes, so Kapp asked him to speed it up because disc jockeys at the time wouldn't play a song longer than that length, Williams often said.
The second time around, "Autumn Leaves" came in at 2:59.
"I owed two months' rent on our apartment," Williams said in the 1996 Times interview. "I had a wife and a baby at the time. My fondest dream for 'Autumn Leaves' was that it would pay the rent. And boy, it's been doing it ever since."
The tunes he played were so familiar "they almost play themselves," the New York Times said in a 1972 review of a performance that included "On a Clear Day," "Hello Dolly" and "The Impossible Dream."
If dissected, Williams' approach could seem "trite and oversimplified," the review said. "There is a cumulative charm to the unadorned simplicity and sincerity of his performance that can eventually win over even the most cynical listener."
The son of a Lutheran minister, he was born Louis Weertz on Oct. 1, 1924, in Omaha and grew up in Des Moines.
At 3, he toddled toward the piano and just started playing, Williams often recalled.
While majoring in piano at Iowa's Drake University, he began developing a style that was a fusion of jazz, classical and pop.
When a school official overheard him playing the show tune "Smoke Gets in Your Eyes," he was expelled because the school had a "classics only" policy, Williams later said.
So he joined the Navy during World War II and won the middleweight boxing championship at his base, in Idaho, the Los Angeles Times reported in 1959.
In the Navy, he developed an aptitude for engineering and earned a bachelor's degree in the subject in 1949 at Idaho State University. He followed it with a master's in music in 1950 at Drake.
While enrolled at the Juilliard School in New York, he won "Arthur Godfrey's Talent Scouts" on television playing a symphonic arrangement of "I Got Rhythm."
Kapp insisted that the pianist "have a name that would stand up anywhere" and changed it to Roger Williams after the founder of Rhode Island.
Williams' Top 10 albums include "Songs of the Fabulous Fifties" (1957), "Till" (1958), "Maria" (1962) and "Born Free" (1966).
In the early 1970s, his popularity faded but he went on to record more than 100 albums and performed in concert as recently as March. He claimed to know 10,000 songs by heart, often fielding requests, and was long featured at the Crystal Cathedral in Garden Grove.
He played for nine U.S. presidents, starting with Harry S. Truman, who requested Bach and Ravel. Truman then played his own version of the Chopin waltz, Williams told the Atlanta Journal Constitution in 2004.
Williams had three children with his first wife but after "one tour too many" the couple divorced, he later said.
After he married Louise De Carlo in 1985, they maintained houses across the street from each other in Encino, an idiosyncrasy night-owl Williams attributed to conflicting schedules. They eventually divorced.
Of his talent, Williams said in 2007 in the Allentown, Pa., Morning Call: "I know I was given a gift, and once in a while I go to church and say, 'Hey, thanks.' I play my religion through my fingers."