Johnson, who was most frequently cast opposite June Allyson and Esther Williams during his MGM heyday, died of age-related causes at a senior residence in Nyack, N.Y., said Wendy Bleiweiss, a close friend.
With his broad smile, red hair and freckled face, the tall onetime Broadway chorus boy personified the wholesome young American man, MGM-style.
"A Guy Named Joe," the 1943 fantasy romantic-drama starring Spencer Tracy as a World War II pilot who is killed in action and returns to earth in spirit form to help novice pilots, provided a breakout, critically acclaimed role for Johnson: He played a young pilot who falls in love with Tracy's girlfriend (played by Irene Dunne).
Johnson had become an MGM contract player only a year earlier, but his road to stardom nearly ended before he ever got in front of the cameras for "A Guy Named Joe."
While he was driving to a screening at MGM with friends in March 1943, a car ran a red light at a Culver City intersection near the studio and smashed into the side of Johnson's convertible, causing it to roll on its side.
With a fractured skull, severe facial injuries, a severed artery in his neck and bone fragments piercing his brain, Johnson underwent several surgeries. He was left with a severely scarred forehead and ametal plate on the left side of his head, which exempted him from military service.
But his near-fatal accident and three-month hospital stay provided the kind of publicity that not even MGM could buy: The fan magazines ate it up. And his 4-F status allowed him to continue his fledgling movie career at a time when many Hollywood stars were in uniform.
For his part, Johnson fought World War II on the big screen, the wardrobe department providing him with a constant change of GI garb.
Among other films of the period in which he played a serviceman are "Thirty Seconds Over Tokyo" (1944), "Two Girls and a Sailor" (also 1944, and the first time he received top billing) and "Week-End at the Waldorf" (1945).
By war's end, he'd later joke, "I'd been in every branch of the service, all at MGM."
By the end of 1945, Johnson also had joined the ranks of the top 10 box-office stars for the first time, placing second (behind Bing Crosby) in the annual exhibitors' poll.
So great was Johnson's bobby-soxer following that the Hollywood columnists dubbed him "The Voiceless Sinatra."
"He handled it very well," Esther Williams told The Times in 2003. "He was probably one of the most charming, gregarious of the stars I worked with.
"If you look at any of his pictures, look at 'Thirty Seconds Over Tokyo.' You'll see why he was a big star, because he had this wonderful, boyish quality with those freckles and that smile. He was so natural."
June Allyson summed up the actor's screen appeal this way: "He was very, very down-to-earth," she told The Times in 2003. "I think he was the man every girl would like to marry. I just loved working with him. He was delightful, he was funny, and he was always prepared."
Johnson's high-flying popularity, however, was short-lived.
By 1946, he had fallen only slightly in ranking at the box office -- from second to third place -- but it was the last time he made the top 10 list.
In January 1947, Johnson married Eve Wynn, the former wife of his close friend, actor Keenan Wynn. Johnson married Wynn, the mother of two young sons, in Juarez, Mexico, only four hours after she had obtained a Mexican divorce. (The marriage, which produced a daughter, Schuyler, ended in divorce in 1968.)