The former plumber from Smithsburg made the admission in a Montgomery County courtroom, sparing the families a trial that was expected to last six weeks.
He was sentenced to six consecutive life sentences for murder and attempted murder - two without parole.
He also gave up his post-conviction rights, meaning he can never request a modification of his sentence.
Pryor stabbed Alison Munson, the mother of his children, to death in December 2007 in her apartment in Halfway. A short while later, he shot Smithsburg police officer Christopher Nicholson to death.
Munson's family objected to the plea agreement. Munson's stepfather told Pryor in court that he wanted him to die. He then turned to Pryor's parents: "We wanted your son to die."
Nicholson's family says they support the death penalty, but feels they have been forced to take the plea due to "political events and choices by the State of Maryland."
In a typed statement, Officer Nicholson's father, Larry Nicholson says, "My family and I believe that in the end God will truly be the one to punish Douglas Pryor, but as for myself, I will gladly trade my place in heaven just to be waiting for him at the gates of hell the day he arrives." To read Larry Nicholson's full statement, click the attachment icon.
Pryor cried while he apologized to all the families. He says it was the worst night he has ever had.
A judge went over the plea agreement with Pryor "line by line" Wednesday night, according to NBC25 reporter Erin Wolfe who was in the courtroom.
It was also revealed in court that Pryor is currently taking anti-psychotic and anti-depression medications.