"This was not the verdict we sought, but we respect the process of the jury system," she said.
But O'Malley said she's glad the jury found true the allegation that Mehserle deliberately used his gun and rejected the claim by his lawyer, Michael Rains, that he believed he was using his Taser stun gun and that the shooting was an accident.
She said Mehserle will face a term of between five and 14 years in state prison when he's sentenced by Los Angeles County Superior Court Judge Robert Perry.
"He's going to prison and will pay for his crime," O'Malley said.
Speaking to reporters in the lobby of the Rene C. Davidson Courthouse in Oakland, where her office is located, O'Malley said, "The jury clearly didn't find that the shooting was an accident."
She said the jury's finding means that jurors believed Mehserle "was acting with criminal negligence and acting recklessly and intentionally using his gun."
O'Malley said Mehserle faces two, three or four years in state prison for his involuntary manslaughter conviction, plus another three to 10 years for using a gun.
She said he will have to serve his sentences consecutively, which means the minimum time he will serve will be five years.
O'Malley denied the claim by Rains and others that her office caved in to community pressure by charging Mehserle with murder instead of a lesser charge such as manslaughter.
"The decision to charge him with murder was based on the evidence, and at the end of the trial we asked for a murder verdict based on evidence that we believe supported that charge," she said.
O'Malley said 16 dedicated employees in her office worked on the case and that her office "presented our case as strongly as it could be presented."
She said, "This case is a tragedy in every respect, and Oscar Grant should never have been killed at the hands of a sworn officer."
"Our profound condolence goes out to the family of Oscar Grant. He shall not have died in vain," she said.
"His memory can be best honored through open, forthright and candid communication without being steered buy violence, judgment or hatred," O'Malley said. "I urge those in our community to use their voices in a nonviolent manner."