After a curacy in Scarborough,[4] she was the incumbent of the Parish of Georgina from 1987 to 1991 and Holy Trinity, 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 of the Diocese of Toronto in 2007.
Nationally, Nicholls has served as a member of the Commission on the Marriage Canon[6] from 2013 to 2016, the co-chair of the Anglican-Roman Catholic Dialogue in Canada[7] from 2012–present, a member of the Faith, Worship and Ministry Committee of the Anglican Church of Canada from 2011 to 2016, first as a member and then as 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, the 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.
[12] On 13 July 2019, Nicholls was elected the first female Primate of the Anglican Church of Canada.
[13][1] She was installed as primate on 16 July during a service at Christ Church Cathedral, Vancouver.