Born in about 1979, Johnston grew up on dairy farm on Mangorei Road in Taranaki.

She attended Mangorei School and New Plymouth Girls' High School before moving south, initially to Otago, then to Wellington to study and work as a visitor host at Te Papa.

[1][2][3][4][5] Johnston has lived and worked in Wellington with roles at a variety of galleries and cultural institutions, including the Adam Art Gallery, City Gallery Wellington, and from 2012 to 2018 was the director of the Dowse Art Museum after roles at the National Library of New Zealand and Boost New Media where she worked in communications and web roles.

[6][7] In 2019, Johnston became the youngest chief executive to head The Museum of New Zealand Te Papa Tongarewa where in 2024 she introduced a $35 entry fee for international visitors.

[9] Johnston was the 2015 recipient of a Winston Churchill Memorial Trust travel grant for researching contemporary museum practices in the U.S.[1][2][10] In 2022 Johnston was living in inner city Wellington apartment with her husband Reuben Friend.