Nicholls was educated at Wycliffe College, Toronto and ordained in 1986. After a curacy at Scarborough, she was the incumbent of Georgina from 1987 to 1991 and Thornhill from 1991 to 2005. She was also the Co-ordinator for Dialogue within the Anglican Church of Canada until her election as Suffragan Bishop in 2007. Internationally, Nicholls serves on a member of the of the , a member of the Anglican Roman Catholic International Commission III, a participant of Indaba and a faculty member of of the Episcopal ChurchCollege of Bishops. Nationally, Nicholls has served as a member of the Commission on the Marriage Canon from 2013 to 2016, the co-chair of the Anglican-Roman Catholic Dialogue in Canada from 2012–present, a member of the of the Anglican Church of Canada from 2011 to 2016, first a member and the chair of the Standing Committee on Religious Orders of the Anglican Church of Canada House of Bishops, the chair of the Primate's Theological Commission of the Anglican Church of Canada from 2008 to 2010, a member of the Task Force on the Role of the Primacy in the Anglican Church of Canada from 2008 to 2010, co-chair of the Ecumenical Health Care Network of the Canadian Council of Churches, a member of the Canadian Christian Jewish Consultation from 2005 to 2008, co-chair of the National Muslim Christian Liaison Committee from 2005 to 2008, staff resource to the Human Life Task Force from 2005 to 2008, member of the Interfaith Task Group of the Canadian Council of Churches from 2005 to 2008, staff resource to the Faith, Worship and Ministry Committee of the Anglican Church of Canada from 2006 to 2008, a member of the Canadian Anglican-Roman Catholic Dialogue from 1989 to 1998, Chaplain to Church House, national office of the Anglican Church of Canada, a General Synod delegate in 2004 and staff to General Synod in 2007, a member of the Partners in Mission Committee of General Synod in 2005 and a member of the Windsor Report Response Task Force of the Anglican Church of Canada in 2004–2005.
Episcopal ministry
Nicholls was consecrated a bishop on February 2, 2008. She served as a Suffragan Bishop in the Anglican Diocese of Toronto, Canada from 2008 to 2016: she was in charge of the Trent Durham area of the Diocese. On February 13, 2016, she was elected coadjutor bishop of the Diocese of Huron — she was the first woman to be elected a bishop in the Diocese — and she automatically succeeded to the role of diocesan Bishop of Huron on November 1, 2016, upon the retirement of Robert Bennett. On July 13, 2019, Nicholls was elected the first female Primate of the Anglican Church of Canada. She was installed as the 14th Primate on July 16 during a service at Christ Church Cathedral, Vancouver.
