Johnston was a founding editor of Gawker Media's Idolator, and she worked for the site until November 2009. She was a lead contributor at Popdust.com in early 2011. In April 2011, she became the music editor of The Village Voice and remained in that position until September 2012.
She currently is an adjunct professor at Boston College, where she served as the 2013–14 Institute for Liberal Arts Journalism Fellow. From 2010 to 2013, she taught at New York University.
Johnston hosts a weekly radio show on Boston College's non-commercial radio station WZBC and has DJed at WNUR and WPRB. She has discussed music and popular culture on NPR, WNYC, WBUR and CBC Radio.