Decatur taught at Mount Holyoke College from 1995 to 2008. He served as an assistant professor of chemistry, then an associate professor of chemistry, and served as department chair from 2001 to 2004. In 2005, he was appointed the Marilyn Dawson Sarles Professor of Life Sciences. He was also an associate dean of faculty for science from 2005 to 2008.
Decatur was a visiting scientist at the Massachusetts Institute of Technology in 2004 to 2005. From 2008 to 2013, he served as dean of the College of Arts and Sciences at Oberlin College. In 2013, he was named president of Kenyon College.
During his final years at Kenyon College, Decatur faced challenges related both to labor organizing and transgender rights. From 2020 to 2022, students expressed concern over the college not recognizing a student worker labor union. In 2022, students demanded that the college publicly support a transgender staff member, Rhea Debussy, who was receiving transphobic threats. When a statement was not issued, students delivered a list of demands to Decatur's office. Following this, a transgender student then wrote an open letter to Decatur and the college community.