Guarnieri was educated at McGill University, where she graduated with a master's degree in English. As part of her thesis, she wrote a book called The cheese on the moon: a collection of short stories. After graduating she worked for the federal government as a spokesperson for Bob Kaplan{{cite news |last=Mulgrew |first=Ian |title=McMurtry, Ottawa argue over delay |publisher=The Globe and Mail |date=1980-06-28 |page=14}} and also as a press liaison office for Stuart Smith, leader of the Ontario Liberal Party.{{cite news |last=Baker |first=Alden |title=Eggleton to replace civic staff with employees on contracts |publisher=The Globe and Mail |date=1980-03-31 |page=5}} In 1981, she worked as an assistant to Toronto Mayor Art Eggleton but left after seven months "because of a disagreement".{{cite news |title=Another adviser quits mayor's office |publisher=The Globe and Mail |date=1981-11-06 |page=59}} In 1984, she worked for Lowther Consulting, a company owned by Jim Coutts who was a principal secretary for Prime Minister Pierre Trudeau.{{cite news |last=McQuaig |first=Linda |title=Coutts office quiet on eve of decision |publisher=The Globe and Mail |date=1984-04-06 |page=M1}} She and her husband live in Mississauga.

Politics

In 1988 Guarnieri was nominated in the new riding of Mississauga East after a fractious contest with another candidate, Armindo Silva. Silva contended that the vote, which Guarnieri won by a margin of 71, was marred by 'irregularities and improprieties'. He vowed to contest the result with the party's nomination board. The party hired police officers to control the crowd due to threats made against Guarnieri.{{cite news |title=Anger, 'improprieties' mark nomination vote |publisher=The Globe and Mail |date=1988-05-16 |page=A4}} The nomination board upheld the vote despite "deeply disturbing" voting irregularities but this decision was overturned by the Ontario party's executive board.{{cite news |last=Smith |first=Dan |title=Mississauga Liberals replaying their bitter nomination contest |publisher=Toronto Star |date=1988-10-16 |page=A11}} A second, less contentious vote was held which Guarnieri won by a much wider margin.{{cite news |last=Johnson |first=William |title=Machine politics takes a mauling |publisher=Ottawa Citizen |date=1988-10-18 |page=A8}}

Guarnieri was elected as Member of Parliament in 1988 in a fairly close race with her Progressive Conservative opponent Laurie Pallett.{{cite news |title=Decision '88: The vote |newspaper=The Globe and Mail |date=November 22, 1988 |pages=C4–C5}} She won all her subsequent elections in Mississauga East and after 2004 in [Mississauga East—Cooksville by wide margins. A longtime Martin loyalist, she was a relatively obscure backbencher during the Chrétien era. Martin named her Associate Minister of National Defence and Minister of State for Civil Preparedness in his first cabinet. She moved to Veterans Affairs on July 20, 2004.

She also served as Chair of the Standing Committee on Human Resources Development and the Status of Persons with Disabilities, Chair of the Standing Committee on Government Operations, and Co-Chair of the Standing Committee on Official Languages. She also served as Parliamentary Secretary to the Minister of Canadian Heritage.

In September 2010, Guarnieri announced that she would not seek re-election due to being diagnosed with multiple sclerosis.{{Cite web|url=https://nationalpost.com/news/canada/House+back+civility/3553362/story.html |archive-url=https://archive.today/20100924043102/http://www.nationalpost.com/news/canada/House+back+civility/3553362/story.html |url-status=dead |archive-date=2010-09-24 |title=House is back, civility is not |date=2010-09-21 |work=National Post Online |access-date=2010-09-21 }}