[6] Marjorie attended St Vincent's College, Potts Point.

Her father, Brian O'Neill, served as a member of the Industrial Relations Commission of New South Wales for over 20 years.

[10][5] She was later preselected to run as the Labor candidate in the Division of Coogee in the 2019 state election.

Dr. O'Neill defeated Bruce Notley-Smith to become the Member for Coogee, in one of two victories for the Labor Party in the 2019 State Election.

[3][4][7][11] O'Neill used her inaugural speech to discuss her heritage, her family, the history of the electorate of Coogee, and her priorities while she holds the seat.