Espinosa vowed to appeal what he called mistakes by the jury and Superior Court Judge Matias Contreras.
Mostrong has an Aug. 14 sentencing hearing scheduled, and he could be facing nearly 16 years in state prison based on an enhancement charge tied to leaving the scene of the accident.
Deputy District Attorney Wayne Robinson said he will seek a state prison sentence.
The fatal accident occurred just before midnight Oct. 7.
Mostrong had been at a "keg party" in the Superstition Mountains and before he and friends went for a drive in his van, with Mostrong at the wheel.
Michelle and a friend had gone for a ride in a golf cart and the cart had stopped on Wheeler on the northbound side of the road.
Mostrong's van struck a motorcycle stopped next to the van, pushing the motorcycle into the golf cart. The van then struck Michelle. She was thrown 107 feet and died at the scene.
Mostrong stopped his van a distance from the crash site and his friends got out and ran from the scene. One friend, Kristoffer Purdy of El Centro, placed Mostrong into the passenger seat of the van and Purdy drove the two away from the scene.
The van suffered a flat tire in the desert and the two then walked to Imperial High School, where they called for a ride.
Nearly 17 hours later, Mostrong turned himself into the California Highway Patrol.
In the five weeks of testimony, the prosecution attempted to prove Mostrong was under the influence when he got behind the wheel and that the crash could have been avoided, particularly because the golf cart had its lights on.
The defense attempted to show Mostrong was not under the influence and that the golf cart did not have its lights on.
It took just over a day for the jury to reach its verdict of guilty on each of the counts.
After the verdict was read and each jury member was polled, Contreras released the jury.
Contreras also credited the jury for its lengthy service to the community. Jurors declined to be interviewed by the media.
With the jury out of the courtroom, Robinson asked that Mostrong be taken into custody immediately and that there to be no bail in his case.
Espinosa argued that Mostrong should be released on his own recognizance, adding he and his family are responsible and have shown that throughout the proceedings.
Contreras then ordered Mostrong be taken into custody without bail.
Mostrong's friends and family were given one last chance to gather around him before Mostrong was shackled by bailiffs and led away. Family members and friends then quickly left the courtroom.
Robinson, as the courtroom emptied, said, "Justice has been served in the community."
Raymond Grady, Michelle's father, said, "I just think that Blake is getting what he deserves for what he did."
Grady said he feels bad for the Mostrong family, but added Mostrong is still alive and they can still be with him.
"He can regain his life. We cannot regain my daughter," Grady said.
Espinosa said he was "shocked" by the jury's decision.
"I can't believe they could disregard the evidence that was there," he said.
Espinosa said he did not see how the jury could find Mostrong was under the influence.
Evidence showed there was no alcohol in his blood when it was sampled 17 hours after the crash. There was marijuana in his system, but the tests showed the marijuana had been ingested as recently as four hours before the test.
Espinosa said the case will be appealed.
"We felt immediately this case would be appealed if you lost it," he said, adding he did not agree with rulings by the judge that did not allow certain evidence to be presented to the jury.
Espinosa added, "This kid no more belongs in jail than the man in the moon."
Robinson said of Espinosa's vow to appeal, "I have no fear (of the verdict) being reversed."
Staff Writer Kelly Grant contributed to this story.
Staff Writer Darren Simon can be reached at 337-4082.