Johannes Mehserle was found guilty on Thursday in the 2009 killing of 22-year-old Oscar Grant. Involuntary manslaughter carries a sentence of two to four years. Sentencing is slated for Aug. 6. Court spokesperson Alan Parachini said the sentence could be "two, or four or six years in state prison."
The jury had a choice between murder and lesser charges of voluntary and involuntary manslaughter.
The jury also found true a sentencing-enhancement that Mehserle personally used a handgun in the commission of a crime.
The panel included eight women and four men. None listed their race as black. Seven said they were white, three were Latino, and one was Asian-Pacific. One declined to state their race.
Mehserle, 28, pleaded not guilty in the fatal New Year's Day shooting when officers arrived at the Fruitvale station while responding to a fight aboard a train.
At least five bystanders videotaped the incident - one of the most racially polarizing cases in California since four Los Angeles officers were acquitted in 1992 in the beating of Rodney King.
During the trial, Mehserle, who is white, claimed that he mistook his handgun for a Taser during the shooting.
During closing arguments, prosecutors said "this is what happens when a police officer acts out of the desire to punish people." Prosecutors also said that Mehserle lost all control when Grant resisted arrest.
Grant's uncle spoke to crowds gathered outside the courthouse after the verdict was read.
"We knew from the beginning that we was at war with the system. We as a family have been slapped in the face by the system that's denied us the right to true justice. We truly do not blame the jury, but we blame the system," said Grant's uncle.
"Though the system has failed us and though we fight continually, but one thing I know is that the race is not given to the swift nor to the strong but to the one who endures to the end," said Grant's mother Wanda Johnson.
"The next test of course is when and how much time the officer, Mr. Mehserle is sentenced to jail. Under traditional, regular notions, he should be going to jail for the rest of his life," said Grant's family attorney John Burris.
LAPD and the L.A. County Sheriff's Department mobilized extra officers as a precaution against potential violence. Demonstrators outside the courtroom in L.A. called for peace after the verdict was read.
"It's calm because nonetheless, he is convicted. We just don't need anybody out here to provoke anything," said one demonstrator.
"We do not want anyone to be injured, to be hurt, property destroyed or lives lost as a consequence of this verdict. One death is enough," said Burris.
The prosecution was seeking to charge Mehserle with first-degree murder in this case, but the judge said that's not possible because there wasn't enough evidence to show that the shooting was premeditated.
The trial was moved to Los Angeles after racial tensions boiled over into violence in Oakland.
The verdict is prompting strong reaction in the Bay area as well as Los Angeles, where many residents have mixed emotions following the guilty verdict.
"I do not believe it's a just verdict. I can see them dropping the murder one, because he's a sworn officer and with all the privileges that comes along with that," said Los Angeles resident Ras Kari. "I think he had submission by the kid and he's got fellow officers, so there's no way that his life is in danger. It's just no way. It's a farce."
"For him to get some type of sentence that is not acquitted, I think it's justified," said another L.A. resident Joseph McNeil.
AP and CNS contributed to this report.