Mr. Williams died at his home in Los Angeles of complications from pancreatic cancer, according to his former publicist Rob Wilcox.
Known as an electrifying stage performer and an adept improviser, Mr. Williams effortlessly switched between musical styles.
Mr. Williams' 1955 hit "Autumn Leaves" was the only piano instrumental to reach No. 1 on the Billboard pop charts. It remains the best-selling piano record of all time, with more than 2 million sold.
Nicknamed the "pianist to the presidents," Mr. Williams played for every commander in chief from Harry Truman to George H.W. Bush. His last trip to the White House was in 2008, when he performed at a luncheon for then-first lady Laura Bush.
Mr. Williams was good friends with Jimmy Carter, with whom he shared a birthday. When the two men turned 80, Mr. Williams played a 12-hour marathon at the Jimmy Carter Library and Museum in Atlanta, with the former president in attendance.
Born Louis Wertz in Nebraska, he started playing piano at age 3. By age 9 he was prolific with several instruments and could play anything by ear.
As a teenager, he was given his own 15-minute radio show on KRNT, which was broadcast live from a Des Moines, Iowa, department store. Later he hosted a program on WHO, where he first met the station's young sports announcer, Ronald "Dutch" Reagan. The two men started a friendship that lasted over 60 years.
Mr. Williams moved to New York to study jazz at the Juilliard School of Music. He won performing contests on the popular radio shows "Arthur Godfrey's Talent Scouts" and Dennis James' "Chance of a Lifetime."
Soon after, Mr. Williams was signed to Kapp Records, where founder Dave Kapp was determined to find a hit for the young prodigy. Producers decided on a shortened arrangement of "Autumn Leaves," which Mr. Williams recalled first clocked in at three minutes and three seconds.
"In those days the disc jockeys would not play a record over three minutes long. So Kapp asked if I could play the thirds a little faster. I did, and it came in at two minutes and 59 seconds," Mr. Williams said, according to Wilcox.
It was an instant hit, and catapulted Mr. Williams to national renown. He followed it up with a string of hits including "Born Free," "The Impossible Dream," "Theme From Somewhere In Time," and "Lara's Theme from Dr. Zhivago."
Mr. Williams became a popular guest on the top television shows of the time including "The Ed Sullivan Show," "The Perry Como Show" and "The Steve Allen Show."
He is the first pianist to be honored with a star on the Hollywood Walk of Fame. He also received the Lifetime Achievement Award from Steinway & Sons.
On his 75th birthday, Mr. Williams played a 12-hour marathon at Steinway Hall in New York City, a stunt he repeated several time in the following years.
In March, Mr. Williams announced on his website that he had been diagnosed with pancreatic cancer.
Mr. Williams is survived by his daughters, Laura Fisher and Alice Jung, and five grandchildren.
