David Johnston has agreed to remain as Governor-General until September, 2017, a two-year extension of his tenure that will see him preside over Canada's sesquicentennial celebrations.

And in one possible outcome of the Oct. 19 general election, Mr. Johnston could also preside over a hung Parliament, with competing claimants for the job of prime minister.

Canada's 28th governor-general "has made remarkable contributions to Canada in his role as the Queen's representative in Canada, performing his duties with dignity, wisdom and aplomb," Prime Minister Stephen Harper said in a statement released on Tuesday by his office. "I look forward to him continuing his fine work in this critical role."

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The announcement did not come as a surprise in Ottawa. Although, by convention, governors-general serve a term of five years, extensions are not uncommon.

Most recently, when Paul Martin was prime minister, he asked Adrienne Clarkson to stay on as governor-general for an extra year, after the 2004 election resulted in an unstable Parliament and a weak minority Liberal government.

Mr. Johnston's term would have ended in October, in the midst of the general election. Not extending his appointment would most likely have meant appointing his successor this summer, which would leave her or him little time to prepare for a possible constitutional confrontation.

In the event of a hung Parliament, Mr. Harper might seek to lead a minority government, assuming the Conservatives win the most seats. The NDP and Liberals might propose that they govern instead, in a coalition.

In Montreal on Tuesday, Opposition Leader Thomas Mulcair reiterated his willingness to form a coalition with the Liberals, if circumstances required it to unseat the Conservatives. Liberal Leader Justin Trudeau has said he would not consider such an arrangement.

Depending on circumstances after the vote, the governor-general of the day could be faced with a request for dissolution or prorogation.

Mr. Johnston has already served for four and a half years. A lawyer by training, he is well versed in constitutional issues. It was in the interests of all sides to have him stay on.

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But by asking Mr. Johnston to continue through the sesquicentennial, Mr. Harper is signalling his high regard for his abilities.

Mr. Johnston will serve as the Queen's representative during the year-long national celebration of Canada's 150th birthday, when his role will be particularly prominent.

Nonetheless, it must have been a difficult decision for Mr. Johnston to accept. He will be 76 when he leaves office, and many people at that age would not welcome the rigours of the national and international tours a governor-general must undertake.

The Conservative government has asked Mr. Johnston to increase his role in economic diplomacy, representing Canada abroad while officials worked to improve trade relations with countries he has visited.

Mr. Johnston has travelled to 31 countries during his tenure, and with this extension is certain to become the most travelled governor-general in Canada's history.

The itinerary for the sesquicentennial will be especially heavy. And it will certainly have crossed Mr. Johnston's mind that governor-general George Vanier died in office in 1967, in the midst of Canada's centennial celebrations.

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But during his years as a law professor and dean, principal of McGill University, president of the University of Waterloo and as governor-general, Mr. Johnston has stressed the importance of duty to community, country and fellow citizens.