The American Civil Liberties Union filed a civil lawsuit in 2008 on behalf of Ronald Smith that argued the lethal injections the state uses are cruel and unusual punishment and violate the right to human dignity.
District Court Judge Jeffrey Sherlock has agreed, pointing to problems such as lack of training for the individual who administers the drug, a discrepancy over whether two or three drugs should be used in the execution and the method used to determine if the inmate is actually unconscious.
"The Montana protocol has problems," said Sherlock in his 26-page judgment.
"All three of these concerns create a substantial risk of serious harm violative of the plaintiff's right to be protected from cruel and unusual punishment."
But Sherlock noted it wouldn't be difficult to bring the protocol into line with what is expected.
"The court notes, theoretically, the legislature and the Department of Corrections can easily make changes to the protocol as are found wanting in this decision. These changes can be made quickly and, if done, the modified protocol could not be found in violation of the Montana Constitution."
Smith's lawyer said the ruling wasn't a knockout but does buy his client some time.
"It's a temporary victory. It was a given," said Don Vernay, who practises law in Albuquerque, N.M. "When you're executing people in a mobile home, you don't really have your act together.
"They'll just have to redo it. The stay remains in place. Judge Sherlock is the one who issued the stay so obviously now there's nothing they can do until they make a new one and then it comes back to court again. So we're years away probably."
Vernay said it could be at least a year or two before Smith faces a potential execution again, but he is hopeful Montana Gov. Brian Schweitzer will intervene in the meantime and grant his client clemency.
Smith, who turns 55 on Friday, is originally from Red Deer, Alta.
He pleaded guilty in 1983 to shooting cousins Thomas Running Rabbit and Harvey Mad Man Jr. in the head with a sawed-off, 22-calibre rifle while he was high on drugs and alcohol. Their bodies were dumped in the woods near East Glacier, Mont.
He refused a plea deal and asked for a death sentence but later changed his mind. Three decades later, and after several execution dates were set and countless legal arguments made, his request for clemency was rejected in the spring by the Montana Board of Pardons and Parole.
It is now in Schweitzer's hands.
Smith's daughter welcomed the news but admitted it wasn't what she had been hoping for.
"I was hoping there was going to be word from the governor," said Carmen Blackburn, who met with Schweitzer over the summer.
"It's good news and not just for us but anybody that is on death row. We haven't had a lot of good news lately so this is really great. I'm shocked this has been dealt with so quickly."
Vernay said the ACLU deserves credit for the victory.
"God bless 'em because they worked hard on it and it's a battle won," he said.
"But the war is ongoing."