Cellucci died Saturday at the age of 65 at his Massachusetts home of complications from ALS, also known as Lou Gehrig's disease.
His term began in 2001 after serving as Massachusetts governor, and he became known for publicly voicing the views of Canada's neighbour and largest trading partner.
Cellucci spoke out on contentious bilateral issues of the day such as Canada's participation in a U.S. missile defence plan, chided Ottawa for not joining the 2003 Iraq war and called for greater military spending.
But Cellucci defended his approach to "public diplomacy," maintaining it was part of what it means to be a U.S. ambassador in a new, security-conscious world.
Foreign Affairs Minister John Baird says Cellucci was a friend of Canada.
"I can attest to the fact he was a great friend to Canada, and we are grateful for his contributions to the bilateral relationship, both as ambassador and as governor of Massachusetts," Baird said in a statement.
Prime Minister Stephen Harper tweeted his family's condolences.
"Laureen and I are saddened to hear of the passing of former MA Gov and U.S. Ambassador to Canada Paul Cellucci. Rest In Peace."
University of Toronto politics Professor Nelson Wiseman said that Cellucci, like all American ambassadors, was at the end of the day a spokesman for the U.S. administration.
"You can't separate the man from either the president and the president's party, and what's going on at that time in the world," Wiseman said in an interview Sunday.
Speaking with The Canadian Press at the end of his term in 2005, Cellucci said living in Ottawa and travelling across Canada bonded him with the country. He said the massive turnout on Parliament Hill in support of America three days after the Sept. 11, 2001 terror attacks was the biggest surprise of his term.
He also defended his approach, which some saw as meddling in Canada's internal affairs.
"Some people still think that we're in a different era, where diplomacy is always done behind closed doors," Cellucci said.
"For us in the United States, that's no longer the case. We have to be speaking not only to the government, but to the people of a country where we are serving in, so that we can explain how we feel, defend the actions we take, advocate for what the United States is doing and for what it stands for."
Current U.S. Ambassador David Jacobson said the embassy's thoughts are with Cellucci's family.
"Ambassador Cellucci served the American people with wisdom, energy, and enthusiasm. He was greatly admired and respected," Jacobson said in a statement.
Cellucci is survived by his wife, Jan, their daughters Kate and Anne, and four grandchildren.
— By Will Campbell in Toronto, with files from The Associated Press
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