She is best known for voicing the human toddler Boo in the Pixar animated film Monsters, Inc.. Mary Gibbs was born in Pasadena, California, to Pixar director and story artist Rob Gibbs (1964–2020) and his wife Susan (née Hollands).

As Gibbs was a toddler at the time she was recorded as the voice of Boo in Monsters, Inc., her parents helped the crew simulate scenarios that would elicit the required audio reactions that would be her lines; recording was difficult nevertheless because she would run all over the Pixar office.

[2] When Pixar's Inside Out (2015) was in production, the audio recordings of Gibbs' screams as Boo were reused for baby Riley.

[3] Gibbs returned to acting in 2022, voicing the character of Janessa Jensen in the audio-drama series Heroes of Extinction for Adventurous Ideas, LLC.

In January 2022, Gibbs spoke on a panel at the Bakersfield Mouse-Con event about the project alongside other actors and producers of the series: Jason Braden, Isaac C. Singleton Jr., Jerry Cornell, C. Andrew Nelson, and Joshua C.