Her lawyers had argued she couldn't be responsible for the death because she wasn't there, and she has remained free while she appealed the 2017 decision.
The trial judge found Carter caused Roy's death when she instructed him to get back in a truck filled with carbon monoxide.
Conrad, then 18, took his own life after filling his parked truck with carbon monoxide from a generator in a auto park in 2014.
They said evidence in the Carter case suggested "a systematic campaign of coercion" in which she "targeted the equivocating young victim's insecurities and acted to subvert his willpower in favor of her own".
The Massachusetts Supreme Judicial Court disagreed, concluding in its ruling that "the evidence was sufficient to support the judge's finding of proof beyond a reasonable doubt that the defendant committed involuntary manslaughter as a youthful offender, and that the other legal issues presented by the defendant, including her First Amendment claim, lack merit", in the opinion written by Justice Scott L. Kafker, The Boston Globe reports.
Prosecutors at trial presented evidence showing that Roy briefly left the vehicle after he began to be overwhelmed by the fumes, but returned after Carter, who spoke with him by phone, urged him to "get back in".
Roy was in his truck and texted Carter that he was having second thoughts but she insisted he "get back in" the truck.
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The high court cited Carter's own words, in text messages to friends after Roy's death, as supporting her conviction. "I mean, you're about to die", she wrote in another. Carter was 17 at the time.
The high court's ruling "has very troubling implications, for free speech, due process, and the exercise of prosecutorial discretion, that should concern us all", he said.
Carter and Roy lived in MA but met in Florida in 2012 while both on vacation with their families. Both teens struggled with depression.
"We can all see from the text messages that Michelle Carter did not force Conrad Roy to kill himself", Marx said at the time. Carter had also been treated for anorexia, and Roy had made earlier suicide attempts.
They also argued she can't be convicted because of her words alone, noting she wasn't with him when he killed himself and didn't provide him with the means to do it.
But on Wednesday, the Massachusetts Supreme Court shot her appeal down.
"The only verbal conduct punished as involuntary manslaughter has been the wanton or reckless pressuring of a vulnerable person to commit suicide, overpowering that person's will to live and resulting in that person's death", Kafker wrote in the ruling.