Closing arguments were heard Tuesday morning in the penalty phase of the trial for 23-year-old John Lewis.
Jurors then began deliberating Lewis' fate. They returned with the verdict after only an hour.
Lewis again apologized to the Cassidy family after the verdict was read.
The jury forewoman said Lewis was given the death penalty because of the aggravating circumstance of killing a police officer in the line of duty.
After court was adjourned, Cassidy's widow spoke to the media.
"There are no winners today," Judy Cassidy said. "It doesn't make us feel any better. We're glad this is over, but we'll go home now and still deal with this."
"Nobody won," said Cassidy's brother-in-law Tony Conti. "There's no prize here. The Lewis family lost their son, he will die in jail, and we lost Chuck."
Lewis entered a surprise guilty plea to a general murder charge after jury selection earlier this month. The jury convicted him of first-degree murder last week.
Cassidy, a 25-year veteran, was shot in the head as he walked in on a doughnut shop robbery in the city's West Oak Lane section on Oct. 31, 2007.
He died the next day.
"I don't know if this sends a message or not. Some people are, unfortunately, idiots and thugs," said Philadelphia Police Commissioner Charles Ramsey. "But, at least, this guy will not be out to hurt anybody else."
Defense lawyers described the baby-faced Lewis as a young man deserving of mercy. They said he lost his father to violence at a young age.
The defense team said it will appeal the death sentence.
"Granted, he should be in prison for the rest of his life for what he did," said attorney Bernie Siegel. "But when a person accepts his responsibility, when a person says 'I acknowledge what I did, I should be put away for the rest of my life,' where's the necessity to execute him?"
Lewis' family left when the verdict was read, but did not talk to the media.