Killebrew died Tuesday of cancer at his home in Scottsdale, Ariz., the Twins announced. He said in December that he was undergoing treatment for esophageal cancer and last week said he had entered hospice care.
He hit 573 home runs during 22 seasons with the Washington Senators, Twins and Kansas City Royals, including eight seasons in which he hit at least 40 home runs. Killebrew helped the Twins reach the World Series in 1965, where they lost to the Dodgers, and he was named the American League's most valuable player in 1969.
A 13-time All-Star, Killebrew was elected to the Hall of Fame in 1984.
"This is a sad day for all of baseball and even harder for those of us who were fortunate enough to be a friend of Harmon's," Hall of Famer and former Killebrew teammate Rod Carew said in a statement. "He was a consummate professional who treated everyone from the brashest of rookies to the groundskeepers to the ushers in the stadium with the utmost of respect. I would not be the person I am today if it weren't for Harmon Killebrew."
At 5 feet 11 and about 210 pounds, Killebrew was a stocky first and third baseman, outfielder and designated hitter who was particularly known for his ability to hit memorably long home runs.
"He hit a ball in Minnesota that went over 500 feet and broke two chairs," former Twins Manager Cal Ermer told the Chattanooga (Tenn.) Times Free Press in 2002.
Paul Richards, then manager of the Baltimore Orioles, said during Killebrew's breakout season in 1959: "Killebrew can knock the ball out of any park — including Yellowstone."
Killebrew hit 42 home runs that season for the Washington Senators, who moved to Minnesota in 1961 and became the Twins.
Killebrew credited his power to growing up in Idaho. "When I was 14, and for the next four years, I was lifting and hauling 10-gallon milk cans full of milk," he told the Washington Post in 1984. "That will put muscles on you even if you're not trying."
Harmon Clayton Killebrew was born June 29, 1936, in Payette, Idaho. A star athlete at Payette High School, Killebrew already had fans in high places. Idaho Sen. Herman Welker touted Killebrew enough to Washington Senators owner Clark Griffith that Griffith sent the team's farm director to Idaho for a scouting trip.
It was raining when Ossie Bluege arrived to see Killebrew play. They talked and Killebrew made it clear he was planning to play football and baseball at the University of Oregon.
But then the weather changed and the game was on.
"I happened to hit a ball over the left-field fence, and I'd been going to that ballpark since I was a small boy and never had seen anyone hit a ball over that left-field fence," Killebrew said during his 1984 Hall of Fame induction speech.
"Mr. Bluege … called Mr. Griffith and he said it was 435 feet or so [into] a beet field, not a potato patch, and he thought that was a pretty good hit for a 17-year-old boy from Idaho."
Killebrew signed with the Senators in 1954 for $30,000. "It really wasn't a $30,000 bonus," he told the Washington Post in 1984. "They gave me the minimum salary of $6,000 a year for three years and on top of that a yearly bonus of $4,000 for three years."
Because of baseball rules at the time concerning players who received bonuses when they signed with a team, Killebrew had to spend two seasons in Washington before he could be sent to the minors. He played sparingly with the Senators until 1959, when he started a remarkable string of powerful seasons.
The right-handed hitter had more than 40 home runs in a season eight times between 1959 and 1970. He hit 25 home runs in 1965 when the Twins won the American League pennant.
A soft-spoken man who was nicknamed "Killer," Killebrew had enjoyed playing in Washington and was apprehensive about the team's move to Minnesota. But "I quickly learned that Minnesota was my kind of place and the fans there were my kind of people and are my kind of people," he said in his Hall of Fame speech.
In the 1968 All-Star game, Killebrew ruptured a hamstring stretching for a throw at first base. He played only 100 games that season, hitting 17 home runs.
"A lot of people thought I was through," he told The Times in 1985. "But that injury was kind of a blessing in disguise for me. I worked harder in the winter than I ever did before, and I was in better shape the next season than I ever was in my life."
Killebrew bounced back to become the league's most valuable player in 1969, hitting 49 home runs and driving in 140 runs. He hit 41 home runs in 1970. The Twins won the American League Western Division title both seasons.
Killebrew retired after playing with Kansas City in 1975 and spent several seasons as a broadcaster, most of them with the Twins.
"He's one of the great hitters of all time," Al Kaline, a Hall of Fame outfielder with the Detroit Tigers, told the Detroit Free Press in March. "He wasn't just a power hitter. Harmon was strong, but he had great hands and wrists and a great strike zone."
Killebrew's survivors include his wife, Nita, and nine children from two marriages, according to the Twins' website. His first marriage ended in divorce. A complete list of survivors was not available.