Valenti continued to handle assignments for Johnson, and, in November 1963, the vice president asked him to help in a politically sensitive campaign visit that President Kennedy planned to make to Texas. The trip would make Valenti an eyewitness to one of America's darkest chapters and abruptly change the course of his life.
On Nov. 22, Valenti was riding six cars behind the presidential limousine as it snaked through the streets of Dallas toward Dealey Plaza. Valenti would later recall that he never actually heard the shots that killed Kennedy but immediately knew something was wrong.
"Suddenly the slow-moving motorcade became the Indianapolis Speedway," he recalled in a Times piece published on an anniversary of the Kennedy assassination. "The car in front drag-raced from 10 mph to over 60. None of us had any idea of what happened."
After Kennedy died, Johnson asked Valenti to join him on Air Force One flying back to Washington. Valenti can be seen crouching on the left in one of the event's defining photographs showing a somber Johnson taking the oath of office on the presidential jet, Jacqueline Kennedy at Johnson's side still wearing her blood-stained dress.
"That act of inscrutable fate changed my life," Valenti said.
Valenti helped write the words Johnson uttered when he addressed the American people for the first time as president, and bunked at the White House until his family arrived from Texas.
Valenti effectively became Johnson's companion, troubleshooter and trusted confidant. Throughout his life Valenti was a loyal defender of Johnson, even as his presidency was crumbling because of the Vietnam War. He compared Johnson to the Greek mythological hero Achilles, seeing him as a talented leader whose flaws brought him down.
In a 1965 speech to the Advertising Federation of America, Valenti uttered a sentence that would hang around his neck like an albatross: "I sleep each night a little better, a little more confidently, because Lyndon Johnson is my president."
Later, when he complained to Johnson that he couldn't escape the quote, Johnson replied: "I don't know what you're fretting about, Jack. Do you know how few presidential assistants say anything memorable?"
Valenti regarded his time with Johnson in Washington as the "summertime" of his life -- the only period when he was doing something that really "counted," he said in his 1976 book about Johnson, "A Very Human President." Washington, he added, is the "ultimate seduction. After that, everything is tasteless passion."
In 1966, two Hollywood moguls, MCA Inc. power broker Lew Wasserman and United Artists' Arthur Krim, were looking for someone to lead their trade group and they approached Valenti. After initially resisting the idea, Johnson gave his blessing.
Valenti left in April, saying he could not turn down the $175,000-a-year post. The new position paid more than six times his $28,000 White House salary. By the time he left the MPAA he was one of the highest paid lobbyists in Washington, reportedly earning $1.35 million annually.
Two years after taking over the MPAA, Valenti and association counsel Louis Nizer devised the ratings system so they could scrap the industry's Hays Code, which for decades placed tight restrictions on movie language and sexual content. The code had such rules as no open-mouth kissing and a requirement that a man and a woman in bed each have one foot on the floor.
"If you wanted to be affectionate, you had to be Nadia Comaneci the gymnast," Valenti later recalled.
One of his first dealings with the code after being hired by the MPAA was to negotiate what language could be used in Mike Nichols' film version of Edward Albee's play "Who's Afraid of Virginia Woolf?" Valenti would not permit the use of some crude language to describe sex, although he approved use of the phrase "hump the hostess." But the almost comical exercise in discretion made it more obvious that enforcement of the code had become virtually impossible.
At the time, Hollywood was facing competition from more daring foreign films and was seeing a new generation of directors push the boundaries as Nichols did with "Virginia Woolf" and Michelangelo Antonioni did with "Blow Up." Valenti abhorred censorship and wanted to do away with the code but knew he needed an alternative to head off any potential restrictions from lawmakers.
Valenti became immersed in the industry's business issues as well. He championed open markets for Hollywood films and in the final years of his tenure was preoccupied with digital piracy, as technology made it easy to create pristine bootlegs of films.
He did misjudge the impact of home video on the business, initially seeing it as such a threat that it was "to the American film producer and the American public what the Boston Strangler is to the American woman at home alone." Instead, home video became a gold mine for studios.