John Ambulance, the Nova Scotia Lung Association, the YMCA and the Canadian Diabetes Association and is currently Past Chair of the Association of Fundraising Professionals' Foundation for Philanthropy in Canada.
Prior to entering the charitable sector, Mercer worked as Executive Assistant to Nova Scotia's Minister of Labour and Housing from 1974 to 1978.
In February 2013, Mercer became a subject of criticism for having spent the most out of any senator in the past year.On January 29, 2014, Liberal Party leader Justin Trudeau announced all Liberal Senators, including Mercer, were removed from the Liberal caucus, and would continue sitting as Independents.
The senators refer to themselves as the Senate Liberal Caucus even though they are no longer members of the parliamentary Liberal caucus.With the Senate Liberal Caucus facing losing official parliamentary caucus status in 2020 with a third of its caucus facing mandatory retirements on their turning age 75, Senator Joseph Day announced that the Senate Liberal Caucus had been dissolved and a new Progressive Senate Group formed in its wake, with the entire membership joining the new group, including this senator.