Michael Dunn, who is white, said he feared for his life when he opened fire on a red sport utility vehicle in a gas station parking lot, killing Jordan Davis, 17 in November 2012.
Jurors convicted Dunn, 47, on four charges, including three counts of attempted second-degree murder against three other teenagers in the vehicle who survived, and of discharging a firearm after asking them to turn their music down.
Prominent civil rights activist Al Sharpton joined other critics in decrying the "disappointing verdict."
Even though the 12-member jury failed to agree on the primary first-degree, or premeditated, murder charge, Dunn still faces a hefty jail term of at least 60 years for firing 10 rounds into the vehicle.
When jurors initially told Duval County Judge Russell Healey they had been unable to reach a unanimous verdict after four days of deliberations, he asked them to try one more time to agree, but they found no resolution.
Healey declared a mistrial on the first-degree murder count.
Replay of Trayvon Martin?
The closely-watched trial has drawn comparisons to the case of neighborhood watch volunteer George Zimmerman, who was cleared of killing black teenager Trayvon Martin after claiming self-defense under Florida's controversial "stand your ground" law.
The verdict also came days after a retired police officer who shot a man for texting in a Florida movie theater was ordered to be held in prison ahead of trial.
Sharpton, an eminent voice in the uproar that resulted from the Martin case, said the Dunn verdict "clearly does not deal with the fact that a life was lost."
Dunn "was convicted of second-degree attempted murder. But he actually killed someone. I think this sends a chilling effect to anybody, particularly people of color," Sharpton told MSNBC television.
Davis's father said Dunn's expected long prison term, means he will "learn that he must be remorseful for the killing of my son, that it was not just another day at the office."
"He was a good kid," Ron Davis told reporters after the verdict.
"There's a lot of good kids out there... And they should have a voice. They shouldn't live in fear to walk around the streets worrying about if someone has a problem with somebody else, that if they get shot, it's collateral damage."
Grassroots initiative Peace Day Philly, meanwhile, tweeted: "We #StandWithLucy, mother of Jordan Davis #DunnTrial - our children, ALL CHILDREN, are precious. Racism kills us all."
Police did not find evidence of a gun in the vehicle and the three surviving teenagers testified that they never threatened Dunn, who insisted he was "in a panic" when he pulled a pistol out of his glove box as one of the teenagers started to get out of their car and come toward him.
The software engineer testified that he kept firing as the car drove away, saying he was afraid that he or his fiancee -- who had rushed out of the gas station shop when she heard the shots -- might get hit by returning fire.
Dunn says he only learned that he had shot and killed Davis after seeing a news alert on his phone once he got back to his hotel.
Unlike Zimmerman -- who did not face charges for weeks after claiming self-defense in the Martin shooting -- Dunn was arrested and charged with murder the day after the shooting.
Gun control advocates say cases like those seen in Florida are the inevitable by-product of relaxed local laws that allow individuals to carry firearms in virtually any setting.
"Stand Your Ground laws put our children, families, and communities at risk," said Moms Demand Action Founder Shannon Watts.
"Children -- like Jordan Davis -- who may simply be in the wrong place at the wrong time are now more likely to die at the hands of the armed and angry. This is unacceptable."