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 started off her career as a bank auditor for one of Canada's largest banks. 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.

In March 2006, Elliott ran successfully 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.

Elliott became the party's Deputy Leader from 2009 until her resignation in 2015.

On June 25, 2014, Elliott announced she would once again be seeking the leadership of the party for a second time. 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.

On December 10, 2015, she was appointed by Minister of Health and Long-Term Care Eric Hoskins as Ontario's first Patient Ombudsman effective as of July 1, 2016. The duties of the non-partisan position included 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." On February 1, 2018, Elliott resigned as Patient Ombudsman to enter the Ontario PC Party leadership election.

Elliott officially announced she was running for leadership on February 1, 2018, via Twitter, to replace Patrick Brown who had resigned as leader after sexual misconduct allegations were made against him. Elliott's campaign slogan was "Ready. Now." and her leadership campaign has focused on her experience compared to the other candidates. Elliott stated she supported the "People's Guarantee" (the existing platform adopted by the PC Party in November 2017) with the exception of a carbon tax, which she opposed. Elliott had the most support from the Ontario PC caucus of any leadership candidate in the race. On March 10, Elliott was eliminated on the third and final ballot of the leadership election, which was won by Doug Ford. Elliott had the most votes and had won the most ridings but did not have the most points and therefore came in second. Elliott initially refused to concede to Ford, citing “serious irregularities” in the race and vowed to “investigate the extent of this discrepancy”. However she conceded to Ford the next day and announced her support for him as party leader.

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 facility for those with special needs), 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.