Marcia Johnson est une dramaturge et actrice née en Jamaïque, émigrée au Canada à l'âge de six ans et actuellement basée à Toronto, en Ontario. Marcia Johnson a étudié le théâtre à l'American Academy of Dramatic Arts en Californie. Ses premiers concerts d'actrice ont eu lieu au Young People's Theatre. Elle s'initie à l'écriture en travaillant au sein de la Company of Sirens, qui se consacre à la création collective, et perfectionne ses compétences en participant à des séminaires d'écriture auprès de la dramaturge américaine Natalie Goldberg au Nouveau-Mexique.

Her second play, Perfect on Paper, which debunks the myth of romantic love propagated by romance novels, was produced at the Toronto Fringe Festival. In 2008, her next three plays enjoyed world premieres: Binti’s Journey, an adaptation of the teen fiction novel, The Heaven Shop by Deborah Ellis about a 13-year-old girl who loses her parents to AIDS (Theatre Direct); Courting Johanna, based on a short story by Alice Munro entitled “Hateship, Friendship, Courtship, Loveship, Marriage” (Blyth Festival, dir. Kate Lynch); and Late (Obsidian Theatre, dir. Marjorie Chan), an expressionist play which portrays the turbulent emotional journey of a woman who has lost her lover.
Serving Elizabeth (a co-production with Thousand Islands Playhouse) had its world premiere in February 2020 at Western Canada Theatre Company: "Politics and pop culture collide with unexpected twists as this new play moves between 1952 Kenya—when an anti-monarchist cook challenges Princess Elizabeth in the moments before she becomes queen—and 2015 London—when a Kenyan-Canadian film student clashes with convention and custom while working on a TV series about the Royals. The tension between people and politics—opposition to imperialism, embedded prejudice and unconscious bias, and an irresistible attraction to the monarchy as presented in popular media" (WCT website). The playwright had a dual role in the production.

In 2006, Marcia Johnson acted in The Real McCoy by Andrew Moodie at Factory Theatre in Toronto and Great Canadian Theatre Company in Ottawa. More recently, she performed at the 2018 Blyth Festival in The New Canadian Curling Club et 1837: The Farmers Revolt.