Born in Lagos State to a former naval officer, Daniel Johnson and his wife Elizabeth, she is the fourth child in the family of seven.

[6] Right after her secondary education, she auditioned for a role in The Maid and subsequently acted in other movies such as Hustlers, Baby Oku in America, War in the Palace.

[9] Mercy Johnson was once banned from acting for being too expensive on 3 November 2013, the movie marketers of Nollywood threatened to prohibit her from the industry due to her high demands.

[10] She and co-Nollywood stars such as Genevieve Nnaji, Omotola Jalade Ekeinde, Richard Mofe Damijo, Emeka Ike, Ramsey Nouah, Nkem Owoh, Stella Damasus and Jim Iyke were prohibited from acting, for reportedly demanding outrageous pay raise per movie.

[19] In October 2019, Mercy Johnson signed an endorsement deal with Chi Limited's Prime brand, Hollandia Evap Milk.