When Whitner was six years old he was hit by an oncoming car after chasing a loose football down the street. He had up to 30 fractures in his legs from the impact and was told that he might not be able to walk again. The oncoming car suffered significant body damage. He spent three months in a full body cast but recovered.
On August 4, 2011, Whitner signed with the San Francisco 49ers. After previously announcing on Twitter that he would be going to the Cincinnati Bengals, he changed his mind and signed with San Francisco. He agreed to a three-year, $11.75 million deal with $4 million guaranteed. After his performance in his first year as a 49er, he was named to the USA Today All Joe TeamDuring the 2011 playoffs, Whitner became famous for making vicious hits like against New Orleans Saintsrunning backPierre Thomas as he was driving for a touchdown. The hit forced a fumble recovered by the 49ers and took Thomas out for the rest of the game.
In 2013, in a game against the St. Louis Rams, Whitner was fined for a hit on receiver Chris Givens. In response to the fine, he indicated that he would be legally changing his last name from "Whitner" to "Hitner", dropping the "W" from his surname. In an appearance on NFL Network's "NFL Total Access" on November 27, 2013, Whitner indicated that his request was to be processed on November 19, but had to be withdrawn when he was unable to honor the judge's requirement that he appear in court. During the same interview, Whitner indicated that he would pursue his name change during the offseason.