Johnston was appointed to the vice-regal post in 2010 and the term is usually five years, although extensions aren't unusual.
Harper says the extension will allow Johnston to take part in many of the events marking the country's 150th birthday in 2017.
"He has made remarkable contributions to Canada in his role as the Queen's representative in Canada, performing his duties with dignity, wisdom and aplomb," Harper said in announcing the extension. "I look forward to him continuing his fine work in this critical role."
Before his appointment as the country's 28th Governor General, Johnston was a lawyer, academic, and university administrator.
He was a professor who became dean of the faculty of law at the University of Western Ontario in 1974.
In 1979, he was named principal of McGill University and in June 1999, he became president of the University of Waterloo.
He is a former president of the Association of Universities and Colleges of Canada and of the Conference des recteurs et des principaux des universites du Quebec. He was the founding chairman of the now defunct National Round Table on the Environment and the Economy.
Johnston is a native of Sudbury, Ont., who studied at Queen's University, Cambridge University and Harvard, where he was also a hockey All-American athlete.
He and his wife Sharon have five daughters and 11 grandchildren.
Also on HuffPost