Elliott was born in Oshawa and grew up in Whitby. She attended the University of Western Ontario where she received her honours Bachelor of Arts degree in history. She graduated from Western Law School and received her Bachelor of Laws degree in 1978. She was called to the Ontario Bar in 1980. She was a founding member and partner of the Whitby law firm Flaherty Dow Elliott & McCarthy, where she practised in real estate, corporate/commercial and estate law.
Elliott received Whitby's Peter Perry Award, an annual recognition of Whitby's outstanding citizen. Prior to that she became a Rotary International Paul Harris Fellow in recognition of her pro bono legal work. She is the co-founder and director of the Abilities Centre, a director and past chair of Grandview Children’s Centre and a director of the Lakeridge Health Whitby Foundation. She was board president of the Durham Mental Health Services (DMHS) and was recognized when it named one of their group homes in her honour (Elliott House). She is a director of Legacy Private Trust and has been a director of the Leukemia & Lymphoma Society and the Toronto Symphony Orchestra.
Elliott was married to former Finance Minister Jim Flaherty until his death on April 10, 2014. She lives in Whitby with her triplet sons John, Galen and Quinn.
In March 2006, Elliott ran as the Progressive Conservative candidate in the provincial riding of Whitby—Ajax in a by-election, replacing her husband who was elected to the federal Parliament. She was re-elected in 2007 in the redistributed riding of Whitby—Oshawa. She was re-elected in 2011 and 2014.
She was the party's Deputy Leader from 2009 until her resignation.
On April 4, 2009, Elliott entered the leadership race to succeed former leader John Tory. She portrayed herself as a centrist alternative to her three right-wing opponents—Hudak, Klees and Randy Hillier. During her announcement she criticised her main rival Tim Hudak. She said, "He really wants to replicate some of the campaigns and some of the solutions that worked in the 1990s. What happened in 1995 is not the solution for 2009." She said that as leader, she would push for a 'sector-by-sector' innovation strategy for Ontario. She also said that she supports nuclear energy and that the Liberal's green energy plan was insufficient for Ontario's needs. Elliott also supported a flat income tax system. At the leadership convention on June 28, 2009 in Markham, Ontario, Elliott placed third in the results behind winner Tim Hudak and runner-up Frank Klees.
On June 25, 2014, Elliott announced she would once again be seeking the leadership of the party. She had the support of at least nineteen of the twenty-eight caucus members but lost to Brown on May 9, 2015 in a two-person race with 38% of the vote. She resigned her seat in the legislature on August 28, 2015, one week before Brown's election to the provincial parliament.
^ Change is from redistributed results.
On December 10, 2015, she was appointed by Minister of Health and Long-Term Care Eric Hoskins as Ontario's first patient ombudsman. Her duties will include acting on behalf of patients who have not had their issues resolved through normal complaint resolution processes. She said, "Ensuring that patients in Ontario's health-care system will now have a strengthened voice is a responsibility I am looking forward to taking on." The position is effective as of July 1, 2016.