Johnston began her teaching career proper when she accepted the post of lecturer in Classics at Princeton University, where she worked from 1987 to 1988. Since then, she has held a number of positions at Ohio State University, including assistant professor of Classics (1988–1995), associate professor of Greek and Latin (1995–2000) and professor of Greek and Latin (2000–). In 2011 she was named the Arts and Humanities Distinguished Professor of Religion at Ohio State, and in 2017 she was named the College of Arts and Sciences Distinguished Professor of Religion. She holds a professorship in Ohio State's Department of Classics.[citation needed]
She was the founding director of the Center for the Study of Religion at Ohio State (2006–2010).[citation needed]
Her scholarly books include The Story of Myth (2018), Ancient Greek Divination (2008), Ritual Texts for the Afterlife: Orpheus and the Bacchic Gold Tablets (2007, with Fritz Graf), Restless Dead: Encounters Between the Living and the Dead in Ancient Greece (1999) and Hekate Soteira (1990). Additionally, she has also been an editor for a number of collections, including Narrating Religion (2017), Religions of the Ancient World: A Guide (2004) and Ancient Religions (2007), and she has authored a number of articles and essays for Classical journals.[2]
In 2023, her first book for the general public, Gods and Mortals: Ancient Greek Myths for Modern Readers, was published.[citation needed]