The decision meant that Mehserle, 28, could be sent to prison for two to four years. Sentencing is set for Aug. 6. The jury was made up of 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 reveal his race.
Grant family attorney John Burris said they were "extremely disappointed" with the verdict.
"This verdict is not a true representation of what happened to Oscar Grant," he said. "This was not an involuntary manslaughter case."
Jonathan Masur, a professor at the University of Chicago Law School, called an involuntary manslaughter conviction "a compromise verdict."
"At least the jury will view it as a compromise verdict," he said earlier. "I'm not sure that the people in Oakland who are upset would be satisfied with involuntary manslaughter."
Legal experts called the case the most racially polarizing trial in the state since four Los Angeles police officers were acquitted in 1992 in the famed Rodney King beating.
Oakland community leaders took steps to try to ensure calm after the verdict, including opening centers for people to vent their feelings.
Mehserle was charged with first-degree murder in the killing, but a judge removed that option last week and ruled that the jury could consider only second-degree murder or lesser manslaughter charges -- or a verdict of not guilty of any charges.
After testimony in the trial ended, the prosecutor last Friday asked a Los Angeles County jury to do what no local jury had done in nearly 30 years: convict a police officer of murder in an on-duty shooting.
The shooting sparked violent street protests in Oakland, where the death of Grant – a grocery store worker and young father from Hayward, Calif., whose criminal record included violent felony convictions and who had been recently released from jail – was seen as a symbol of long-simmering tensions between minorities and law enforcement. He became a martyr of sorts, his image plastered on buildings and storefront windows across the city, his name memorialized in hip-hop songs and murals.
In Oakland, police moved to prevent a repeat of the rioting that occurred after grainy videos of the shooting were made public. At the time, there were more than 100 arrests, the majority involving what police called "professional rioters and agitators" from outside the Oakland area.
In anticipation of the verdict, officers brushed up on crowd control procedures and worked 12-hour shifts. In Los Angeles, already-tight security around the courthouse was beefed up and calm prevailed.
In his closing argument, Alameda County Deputy District Attorney David Stein said police officers were supposed to protect and serve, not abuse their authority. "We also entrust them with our lives and our fellow citizens,'" he said. "Oscar Grant represents one of those citizens."
But defense attorney Michael Rains called the shooting a tragic accident, saying his client had inadequate training. Mehserle testified that he accidentally grabbed his gun instead of reaching for his Taser. Witnesses testified that Mehserle appeared to be in shock after shooting Grant.
"On Jan. 1, 2009, Johannes Mehserle fired a single gunshot and it brought him to this place before you, and you are his shot at justice," Rains told jurors, using a line from the 1982 legal drama "The Verdict," starring Paul Newman.