Kaplan is probably best known for his 1980-84 stint as Canada's solicitor general, when he presided over the demise of the RCMP's disgraced security service and the creation of the Canadian Security Intelligence Service.
He also ushered in the Young Offenders Act.
Interim Liberal leader Bob Rae disclosed Kaplan's death via Twitter.
"So sorry to advise of Robert Kaplan’s passing," Rae tweeted. "He was a good friend to many who contributed much to Canadian politics and public life."
In an email sent Monday night, Rae called his colleague an "articulate, intelligent, fine man."
"A caring and thoughtful man, (he) worked hard as Solicitor General," Rae said. "Will be sorely missed."
A funeral is to be held Tuesday at Benjamin's Memorial Chapel in Toronto.
Kaplan was first elected in 1968 as an MP for the Toronto riding of Don Valley.
He was defeated in 1972 and switched to York Centre in 1974, a riding he held for almost 20 years before retiring from politics in 1993.
He took on the solicitor general's post in 1980, a time when the RCMP was reeling from scandals which had implicated the force's security service in illegal activity. CSIS was created in 1984 to take over responsibility for security intelligence.
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