A Progressive Conservative, she was first elected to the Legislative Assembly of New Brunswick in the 1999 provincial election defeating Liberal candidate Maurice Richard by just over 100 votes – the closest result of the election. She faced Richard again in the 2003 election and defeated him by 321 votes. Poirier joined the New Brunswick cabinet following the 2003 election as minister responsible for the Office of Human Resources and retained that post until a cabinet shuffle in early 2006 when she became Minister of Local Government and Minister responsible for Aboriginal Affairs.
She was re-elected in 2006 in which her party"s government was defeated and went into opposition.
On January 29, 2010, she was appointed to fill a vacant New Brunswick seat in the Canadian Senate by Prime Minister Stephen Harper. Her appointment became effective February 28, 2010.
Unlike the other four senators named on January 29, Poirier"s appointment was delayed a month so that the government of New Brunswick would not have to call a by-election to replace her just months before the 2010 provincial election.