The officer's wife sat stoically, arms folded, as the long verdict was delivered.
In the state of IL, second-degree murder carries a four- to 20-year prison sentence but can also result in four years of probation instead of prison.
Jurors found Van Dyke not guilty of official misconduct.
James Thomas O'Donnell, a pharmacologist who testified in the trial, said that someone with as much of PCP in his system as McDonald had in his bloodstream could have a "feeling of omnipotence", according to The Associated Press.
Police have been put on 12-hour shifts to bolster available numbers by up to 4,000 officers. He had been on the force for 13 years when the shooting happened.
Police spokesman Anthony Guglielmi said Friday that he could not go into specifics as to what the threats were.
The Chicago Marathon is due to be staged this Sunday, placing police and the city under greater strain.
Chicago is bracing for protests.
Assistant prosecutor Jody Gleason argued that Van Dyke had no right to fire even one shot, let alone 16, including several that struck the teen in the back, and while he was already on the ground.
Van Dyke faced two counts of first-degree murder, 16 counts of aggravated battery and one count of official misconduct. She said she was a little disappointed that Van Dyke wasn't found guilty of first-degree murder but said his conviction was a step toward "what we've been fighting for forever and needs to be an example".
Pep Guardiola Gives Update On Kevin De Bruyne Injury
But I said it before, always someone is telling me, "Good luck for Sunday", and I think always, unfortunately, we need it. And that record has led to some pundits claiming that Klopp has worked out how to beat Guardiola sides.
But a legal expert explained that the 40-year-old Van Dyke is likely looking at less than 10 years in prison for killing the teen rather than many decades because jurors opted to convict him of second- and not first-degree murder.
The city paid out a multi-million-dollar settlement to McDonald's family, and Van Dyke was indicted for the shooting in December of 2015. Police officers worked to hide what happened on the scene, and in initial reports they stated that McDonald had lunged at them with his knife. Though he often appears unflappable, and his questioning of witnesses is typically a quiet march through the evidence he wants to cover, McMahon has also shown a dramatic side, as he did in his opening statement to jurors when he rapped his knuckles on the lectern once for every shot Van Dyke fired. The jury consisted of eight women and four men, seven of them white, one Black, three Hispanic and one Asian.
A judge has decided against taking a Chicago police officer on trial for murder into custody for being late to a court hearing.
Before the shooting, Jason Van Dyke never saw his name in the paper.
"Someone needed to arrest Laquan McDonald, not stop him with a hail of gunfire", McMahon said in his closing argument on Thursday.
"Where was [McDonald] actually causing an issue that Jason Van Dyke thought that he needed to use deadly force?"
They also said he had exaggerated McDonald's actions that night to justify the fatal shooting.
It makes flawless sense that Daniel Herbert should specialize in representing police officers in everything from civil rights lawsuits to divorce cases, and for arrests for crimes ranging from drunken driving to murder. You can determine what is a murder.
The reaction in Norris Friday was muted: the roughly three dozen people standing around the TV nodded or quietly sighed in relief after the verdict was announced.
However, anti-police fliers posted around the city and posts on social media had led police to be watchful in northwest and southwest neighbourhoods where many police officers live.