His announcement Friday in Burnaby, B.C., came after the federal government tabled Bill C-54 in Parliament earlier in the day.
Harper said the changes would create a new legal designation to protect the public from an accused person designated as "high-risk non-criminally responsible."
While making the announcement, the prime minister was joined by Justice Minister Rob Nicholson and Heritage Minister James Moore, the MP for Port Moody-Westwood-Port Coquitlam.
The changes, Harper said, would also ensure that victims would be notified when a "high-risk non-criminally responsible" accused person was discharged.
In other words, Harper said, the changes would "help ensure we have a justice system that puts the rights of victims first."
One such victim is Darcie Clarke, whose ex-husband Allan Schoenborn killed their three children in Merritt, B.C., in 2008.
Schoenborn was found not criminally responsible because of a mental disorder, and by November 2012, Schoenborn was eligible to apply for day passes.
Clarke's family was in the room during the prime minister's announcement.
Stacy Galt, Clarke's cousin, said the proposed changes would remind "the courts and provincial review boards that victims matter."
"Our current system allows a triple-child murderer to be given fast freedoms, and the mother of his victims to have no say," Galt said.
Clarke's cousin said "there's an imbalance in our [justice] system and this legislation corrects this."
Galt, on behalf of Clarke and her entire family, urged all members of Parliament to "unanimously" pass the legislation.
Pleased with government's quick action
Also hoping for the bill's swift approval is Carol de Delley, whose 22-year-old son, Tim McLean, was beheaded by another man aboard a Greyhound bus in Manitoba in July 2008.
"I'm very pleased that this has come about as quickly as it has," de Delley said.
"I've honestly felt, like everybody else, with government it's going to take forever and a day for anything to change."
Vince Li was declared not criminally responsible in McLean's death, after psychiatric evidence determined he is a schizophrenic who suffered a major psychotic episode.
Li has been committed to a mental hospital north of Winnipeg. Last year, a review board allowed Li to go on short, escorted outings from the hospital, sparking public outrage.
Getting tough on mental illness?
However, mental health organizations like the Schizophrenia Society of Canada say the federal government is getting tough on mental illness in its bid to crack down on crime.
Chris Summerville, the society's chief executive officer, said Canadians should know that mentally ill offenders can respond to treatment and recover.
"The public did not hear today that when [not criminally responsible] offenders are released, that 93 to 97 per cent never reoffend again," he said.
Summerville added that only three per cent of those with mental illness come into conflict with the law.
The schizophrenia society was not consulted on the proposed legislation, he said.
Opposition MPs react
In an interview on CBC News Network's Power & Politics, NDP justice critic Francoise Boivin said she was "disappointed" with the proposed changes.
She told guest host Hannah Thibedeau the proposed legislation does not guarantee that the so-called imbalance in the justice system will be corrected.
Boivin said the "ugliness" of the crime ought not to be confused with the factors going into the decision to designate an accused as a "high-risk non-criminally responsible."
Sean Casey, the Liberal associate justice critic, told Thibedeau the proposed changes were more about politics than actual evidence-based policy.
"It's all about playing to the [Conservative] base," said Casey.
The proposed changes would also enhance the safety of victims, the prime minister said.
Under the new designation, an accused person would not be eligible for unescorted passes and would also have to wait three years before getting a review by a mental health board.
The high-risk designation would not affect the access to treatment by the accused, Harper said.
The review boards would also have to follow stricter guidelines and only the courts would have the power to revoke the high-risk non-criminally responsible designation.
The changes are also expected to include a bigger role for victims at every stage as soon as trials are completed.
The Conservative government has been signalling changes to the treatment of mentally ill high-risk offenders for months, in the wake of high-profile cases in British Columbia, Alberta, Manitoba and Quebec.Suggest a correction