His nickname, "Pronk", was given to him by former teammate Bill Selby during spring training of 2003 when people sometimes referred to him as "the Project" and other times "Donkey" for the way he looked when running the bases.

Hafner was born in Jamestown, North Dakota, in 1977 and attended a small high school in Sykeston, which did not have a baseball program.

[7] Hafner made his major league debut that day against the Detroit Tigers, pinch hitting for Michael Young in the ninth inning and striking out.

[10] On December 6, 2002, the Rangers traded Hafner to the Indians along with Aaron Myette for catcher Einar Diaz and right-handed pitcher Ryan Drese.

[15] He hit his first career grand slam in the Indians' home opener on April 12, against Kyle Lohse of the Minnesota Twins.

[10][18] The American League named Hafner Player of the Month for June, during which he posted a .345 batting average with 10 doubles, eight home runs, and 29 RBI in 24 games.

To end the season, Hafner hit home runs in six straight games from September 18–24, the second longest such streak in Cleveland history.

[22] After the season, the Cleveland chapter of the Baseball Writers' Association of America (BBWAA) named him Indians Man of the Year[23] and he finished fifth in the AL Most Valuable Player voting.

[27] On July 7, Hafner became the first player in Major League history to hit five grand slams before the All-Star break and passed Al Rosen in the team's season record book when he homered off Kris Benson of the Baltimore Orioles.

He joined Hall-of-Famer Ernie Banks of the 1955 Chicago Cubs, Jim Gentile of the 1961 Orioles and Don Mattingly of the 1987 Yankees as the only players to hit at least five grand slams in a season.

[28] A little more than a month later, on August 13, Hafner tied Mattingly's single-season record when he hit his sixth grand slam of the season, off Luke Hudson of the Kansas City Royals.

[29] His league-leading 13 home runs and 30 RBI, combined with his .361 average in the month of August, earned Hafner AL Player of the Month—the second time he had been honored as such in his career.

The Indians signed Hafner to a four-year, $57 million contract extension during the All-Star break, keeping him in Cleveland through the 2012 season.

When he did play, Hafner's performance was the worst of his career, finishing his abbreviated season with a .197 batting average, five home runs, 24 RBI and a .628 OPS.

[32] Hafner hit another walk-off home run on July 7, 2011, a grand slam off Toronto Blue Jays reliever Luis Pérez.

[34] On April 15, 2012, Hafner hit a home run off Kansas City Royals starting pitcher Luis Mendoza that was estimated at having traveled 456 feet.