He continued to turn out hits in the 1960s and '70s, including "Can't Get Used to Losing You," "Dear Heart," "Charade," "Music to Watch Girls By" and "(Where Do I Begin) Love Story."
The son of a railroad mail clerk, Williams was born Dec. 3, 1927, in Wall Lake, Iowa. As a boy, he began singing with his three older brothers — Bob, Don and Dick — in the local Presbyterian church choir.
"The very first time he heard his four sons harmonize together, my dad became a man with a dream and a mission in life, convinced that we had a future as professional singers," Williams wrote in his 2009 memoir "Moon River and Me."
With their father as their manager and "driving force," the Williams Brothers landed a 15-minute show every weekday morning on a Des Moines radio station in 1936 when Andy was 8. They had similar success after moving to Chicago in 1941, and later Cincinnati.
The family headed west in early 1944 and the Williams Brothers soon got two big breaks in Hollywood: Bing Crosby hired them to do the backing vocals on his 1944 hit "Swinging on a Star" and MGM signed them to a contract.
They appeared as a quartet in half a dozen forgettable movies, including "Kansas City Kitty." While his brothers were in the service during World War II, Andy sang in a few other close harmony groups before spending six months in the merchant marine toward the end of the war.
In 1947, the Williams Brothers teamed with singer-comedienne Kay Thompson to form a critically acclaimed nightclub act that toured the country for a number of years. The Williams Brothers split up in 1953; his older brothers were tired of travel and were starting their own families.
Williams spent more than two years as a regular on Steve Allen's "Tonight" show, singing and appearing in comedy sketches, and then hosted summer replacement variety series from 1957 to 1959 before launching his long-running NBC variety show in 1962.
In 1961, he married Paris-born Claudine Longet, then a young Las Vegas dancer, with whom he had three children, Noelle, Christian and Robert, named after their close friend Sen. Robert F. Kennedy.
The Williamses were among those gathered in Democratic presidential candidate Kennedy's suite at the Ambassador Hotel in Los Angeles on June 4,1968, when he won the California presidential primary. They were headed downstairs to meet with Kennedy when the senator was fatally shot shortly after midnight.
At Kennedy's funeral, Williams sang "The Battle Hymn of the Republic." He later called it "probably the hardest thing I ever did."
"It was thrilling and heart-wrenching," he told the Chicago Sun-Times in 2000. "I remembered he loved hearing that song at rallies and such wherever his campaign travels took him. I started singing it and then everybody in the church just joined in, and it was the most moving moment I have ever experienced."
Williams and Longet were divorced in 1975, and the next year a dark chapter of his personal life began when Longet was charged with the shooting death of her boyfriend, skiing champion Vladimir "Spider" Sabich.
He later said he received only positive feedback for supporting his ex-wife, who claimed the pistol went off by accident. She was found guilty of criminally negligent homicide, a misdemeanor, and sentenced to 30 days in jail in 1977.
"People respected me for standing by her," Williams said in 1981. "I didn't think I did anything special — I just did what I had to do. I was there to take care of my kids and make sure they were all right, and because I love my ex-wife. I wasn't worried about my image. That was the last thing I cared about."
When Caesars Palace opened in Las Vegas in 1966, Williams was the first performer to headline, and he remained a star attraction there for two decades.
An avid golfer, he hosted the variously named Andy Williams San Diego Open for many years.
In 1992, while continuing to perform across the country, he opened the Andy Williams Moon River Theatre in Branson, when it was mainly a hub for country music.
"I didn't want to pretend I was a country singer," he told the Vallejo (California) Times-Herald in 2004. "I was going to be myself. I wanted to make a theater that represented what I felt I was — classy, sophisticated in a way. So I built a theater like that."
Even in his 80s, Williams retained a singing voice that, as one British writer put it, was "still as smooth as silk lingerie."
Williams' survivors include his second wife, Debbie; his three children, Robert, Noelle and Christian; six grandchildren; and brothers Don and Dick.