Cossey smiled when his verdict was read and his father and sister left the court. The victims' families shouted in the public gallery.
Cossey was also issued a three strikes warning as manslaughter is a qualifying violence offence. Both Cossey and Jones were granted bail by Justice Hinton.
Along with other conditions, Cossey was given a 24-hour curfew, while Jones is allowed to continue to work and has a curfew of between 7pm and 7am.
The family of Jason McCormick Ross said the only thing he did that night was to "trust a new acquaintance, Lance Robinson, with his life and become an innocent passenger in this man's vehicle.
We are relieved with the outcome today and we [hope] that the boy racer community reflects on this guilty verdict and comes to some conclusions that their illegal and criminal behaviour on our roads is not tolerated, and takes innocent, precious lives like that of my son."
They said they were not impressed with Jones' not guilty verdict for manslaughter.
"Stephen Jones' lesser charges in our minds in no way reflects his innocence but is merely a result of a jury unable to convict him without reasonable doubt."
Cossey and Jones had denied four charges of manslaughter by means of illegally racing.
They were also charged with operating a motor vehicle in a race or unnecessary exhibition of speed causing injury and failing to stop.
Jones faced an additional charge of attempting to pervert the course of justice at Hamilton on June 28, 2016, after editing and shortening a film he took as the crash happened.
Cossey's lawyer Phil Morgan QC earlier told the jury the unquestionable issue was that Robinson drove dangerously or recklessly and that he caused the deaths of the deceased.
"But it's stretching the law too far to say it's Mr Cossey who is guilty of manslaughter."
Jones' lawyer Russell Boot told the court on Friday that his client was merely a passenger in the car and did nothing more than film the Skyline's overtaking manoeuvre and then give it to police to help them.
The blame for deaths lay with Robinson, who was driving with alcohol, methamphetamine and cannabis in his system, he said.
The pair were convicted by Justice Anne Hinton and will be sentenced on April 20.