 She was educated at Trinity College, Dublin, and later at the London School of Economics.A social worker by profession, Nicholson worked at the Hospital for Sick Children in Toronto, taught at George Brown College and the University of Toronto and also worked at Ontario Correctional Services and as a founding member of the Ontario Commission on the Status of Women.She entered politics in the 1974 federal election, defeating Paul Hellyer in the riding of Trinity in Toronto and was re-elected three times as a Liberal.