She was elected to that post on November 5, 2015—becoming the third woman to hold the post. Ambrose was the third female leader of Canada's major centre-right party. The first was former Prime Minister Kim Campbell who led Canada's now defunct Progressive Conservative Party of Canada, and the second was Deborah Grey, of the former Canadian Alliance. She is also the third woman to be Opposition Leader, after Grey and the NDP's Nycole Turmel. All three of them served in an interim capacity. Under the party constitution, as interim leader she was not eligible to run for the leadership at the subsequent Conservative Party of Canada leadership election. On November 13, 2015, Ambrose responded to the terrorist attacks in Paris committed by the Islamic State of Iraq and the Levant. Ambrose stated, "The fight against ISIS (ISIL) requires a strong humanitarian response, but also a military response ... It's important that we remain resolute and support our allies." Ambrose supports an inquiry into missing and murdered Indigenous women. In February 2017, while still a sitting MP, Ambrose introduced Private Member’s Bill C-337, "An Act to amend the Judges Act and the Criminal Code," also known as the JUST Act. If passed, this bill would require judges in Canada to undergo training on sexual assault law, involving education on rape myths, stereotypes about victims, and the impact of trauma on memory. The JUST Act gained widespread support from experts and victim advocacy groups and was subsequently passed unanimously by the House of Commons. However, the bill did not pass third reading in the Senate, as the Senate rose for Summer 2019 without voting on the JUST Act, resulting in expiration on the order paper, as the order paper is wiped clean for the October 2019 federal election. In May 2017, Ambrose announced to her caucus that she would leave federal politics at the end of the spring session of Parliament, in June 2017, several weeks after her successor as Conservative leader is chosen. Ambrose has stated that though she supports the merger of the Progressive Conservative Association of Alberta and the Wildrose Party, she will not be a candidate for the leadership of the proposed new United Conservative Party of Alberta. She joined the Canada Institute at the Woodrow Wilson International Center for Scholars, a Washington, D.C. based think tank, as a visiting fellow focused on Canadian–American trade and to lead the organization's campaign to educate officials in both countries about the benefits of an integrated North American economy. She was also appointed a member of the NAFTA advisory council set up by the Trudeau Liberal government in August 2017.