Graves died of an apparent heart attack outside his Los Angeles home, publicist Sandy Brokaw said. He would have been 84 this week.
Graves had just returned from brunch with his wife and kids and collapsed before he made it into the house, Brokaw said. One of his daughters administered CPR but was unable to revive him. Graves' family doctor visited the house and believed he had a heart attack, Brokaw said.
Although Graves never achieved the stardom his older brother, James Arness, enjoyed as Marshall Matt Dillon on TV's "Gunsmoke," he had a number of memorable roles in both films and television.
Normally cast as a hero, he turned in an unforgettable performance early in his career as the treacherous Nazi spy in Billy Wilder's 1953 prisoner-of-war drama "Stalag 17."
He also masterfully lampooned his straight-arrow image when he portrayed bumbling airline pilot Clarence Oveur in the 1980 disaster movie spoof "Airplane!"
Graves appeared in dozens of films and a handful of television shows in a career of nearly 60 years.
The authority and trust he projected made him a favorite for commercials late in his life, and he was often encouraged to go into politics.
"He had this statesmanlike quality," Brokaw said. "People were always encouraging him to run for office. But he said, 'I like acting. I like being around actors."'
Graves' career began with cheaply made exploitation films like "It Conquered the World," in which he battled a carrot-shaped monster from Venus, and "Beginning of the World," in which he fought a giant grasshopper.
He later took on equally formidable human villains each week on "Mission: Impossible."
Every show began with Graves, as agent Phelps, listening to a tape of instructions outlining his team's latest mission and explaining that if he or any of his agents were killed or captured "the secretary will disavow any knowledge of your actions."
The tape always self-destructed within seconds of being played.
The show ran on CBS from 1967 to 1973 and was revived on ABC from 1988 to 1990 with Graves back as the only original cast member.
The actor credited clever writing for the show's success.
"It made you think a little bit and kept you on the edge of your seat because you never knew what was going to happen next," he once said.
He also played roles in such films as John Ford's "The Long Gray Line" and Charles Laughton's "The Night of the Hunter," as well as "The Court-Martial of Billy Mitchell," "Texas Across the River" and "The Ballad of Josie."
Graves' first television series was a children's Saturday morning show, "Fury," about an orphan and his untamed black stallion. Filmed in Australia, it lasted six years on NBC. A western, "Whiplash," also shot in Australia, played for a year in syndication, and the British-made "Court-Martial" appeared on ABC for one season. In his later years, Graves brought his white-haired eminence to PBS as host of "Discover: The World of Science" and A&E's "Biography" series.
He noted during an interview in 2000 that he made his foray into comedy somewhat reluctantly.
Filmmakers Jim Abrahams and David and Jerry Zucker had written a satire on the airplane-in-trouble movies, and they wanted Graves and fellow handsome actors Lloyd Bridges, Leslie Nielsen and Robert Stack to spoof their serious images.
All agreed, but Graves admitted to nervousness. On the one hand, he said, he considered the role a challenge, "but it also scared me."
"I thought I could lose a whole long acting career," he recalled.
"Airplane!" became a box-office smash, and Graves returned for "Airplane II, The Sequel."
Born Peter Aurness in Minneapolis, Graves adopted his grandfather's last name to avoid confusion with his older brother, James, who had dropped the "U" from the family name.
He was a champion hurdler in high school, as well as a clarinet player in dance bands and a radio announcer.
After two years in the Air Force, he enrolled at the University of Minnesota as a drama major and worked in summer stock before following his brother west to Hollywood.
He found enough success there to send for his college sweetheart, Joan Endress. They were married in 1950 and had three daughters - Kelly Jean, Claudia King and Amanda Lee - and six grandchildren.
Graves credited the couple's Midwest upbringing for a marriage that lasted more than 50 years in a town not known for long unions.
"Hollywood or New York ... can be very flighty and dangerous places to live, but the good grounding we had in the Midwest ethic I think helped us all our lives," he said.