He described how Roy had climbed out of the truck as it was filling with toxic gas and told Carter he was scared, but she urged him to get back in.
"She did not issue a simple additional instruction: Get out of the truck," the judge said.
He said Carter had a duty to call someone for help when she knew Roy was attempting suicide. Yet she did not call the police or Roy's family.
It was apparent from evidence that Roy was struggling with his issues and trying to find a way to deal with them, the judge said.
"His research was extensive. He spoke of it continually. He secured the generator, he secured the water pump. He located his vehicle in an unnoticeable area. He commenced his attempt by starting the pump," the judge said.
"When Ms Carter realises Mr Roy had exited the truck, she instructs him to get back in the truck, which she has reason to know is becoming a toxic environment."
The judge said Carter's instructions constituted "wanton and reckless conduct".
Sobs broke out in the courtroom when the judge announced the guilty verdict.
The judge ruled that Carter can remain free on bail until her sentencing on August 3. She could face up to 20 years in prison.
Carter has been prevented from texting and using Facebook or Snapchat ahead of her sentencing. She was also ordered not to have any contact with Roy's family.
Roy's father, Conrad Roy Jr, gave a brief statement following the verdict to thank police and prosecutors.
"On behalf of our entire family, I would like to say how thankful we are to Judge Moniz and the prosecution team - mainly Katie Rayburn, Maryclare Flynn, and Scott Gordon - and the Plainville Police Department and the Massachusetts State Police," he said.
"This has been a very tough time for our family, and we'd like to just process this verdict that we're happy with."
The sensational trial raised questions of whether words can kill and offered a window into teen depression and suicide through text messages and Facebook communications.
Prosecutors had argued during the high-profile trial that a then teenage Carter pressured Roy to take his own life through a torrent of text messages.
"I thought you wanted to do this. The time is right and you're ready, you just need to do it. You can't keep living this way," Carter wrote in one message.
"Everyone will be sad for a while, but they will get over it and move on. They won't be in depression I won't let that happen."
During the trial, prosecutors said she told Roy to "get back in" his truck when he became frightened while trying to kill himself with carbon monoxide.
Carter's lawyer argued that Roy had attempted suicide previously and made his own decision to take his life. He also said that she initially tried to talk him out of it.
The judge disagreed on Friday, saying he did not take into account in his verdict Roy's previous attempts at suicide.
An involuntary manslaughter charge can be brought in Massachusetts when someone causes the death of another person when engaging in reckless or wanton conduct that creates a high degree of likelihood of substantial harm.
Roy's handwritten suicide note was shown in court on Tuesday as prosecutors delivered their closing arguments. In the note addressed to Carter, Roy had thanked her for her "effort and kindness".
"I love you and greatly appreciate ur effort and kindness towards me," Roy wrote in a handwritten note in a spiral notebook. "I'll forever be in your heart and we will meet up someday in Heaven."
The court also released Roy's suicide note to his father, in which he expressed his love and his feelings of inadequacy.
"I'm sorry I wasn't the boy you wanted. I can't take the pain," Roy wrote. "I did this to finally be happy."
On Monday, psychiatrist Dr Peter Breggin testified for the defence saying that Carter was a "very troubled youngster" who suffered from depression.
At the time of Roy's death, Carter was taking Celexa, an antidepressant Breggin said targets the brain's frontal lobe, which controls empathy and decision-making.
Breggin said Roy talked about how he wanted to kill himself with a younger and emotionally troubled Carter. He said Carter eventually endorsed Roy's wishes.
The psychiatrist said he reviewed all the text messages and Facebook conversations between Carter and Roy. He said that beginning in 2012, Roy told Carter he wanted to kill himself and said there was nothing she could do to stop him.
"He goes on and on for hours and hours, and pages and pages," Breggin said of Roy's communications with Carter about killing himself at a time when Breggin says Carter is "a little girl" overwhelmed by her boyfriend's unceasing focus on suicide.
In several messages in 2012 that were shown to the court, Roy told Carter that Satan wanted him in hell on a certain date and it was his time to go.
But the tone of the texts between the pair shifted in 2014. Prosecutors focused on a series of text messages Carter sent Roy in the days before he killed himself.
"So I guess you aren't gonna do it then, all that for nothing. I'm just confused like you were so ready and determined," Carter wrote to Roy the day of his suicide.
"You can't think about it. You just have to do it. You said you were gonna do it. Like I don't get why you aren't."
She also instructed him in various texts to "go somewhere in your truck" and "go in a quiet parking lot".
Carter and Roy met in Florida in 2012 while visiting relatives. Their relationship largely consisted of text messages and emails.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
