Before attaining a faculty job, Johnson was a National Science Foundation (NSF) post-doctoral fellow at the Institute for Astronomy, a part of the University of Hawaiʻi.

[11] In 2012, Johnson's team discovered three small rocky exoplanets in a red dwarf star system observed with the Kepler space telescope.

[14] Johnson is the founder of the Banneker Institute, a summer program hosted at the Center for Astrophysics | Harvard & Smithsonian.

It has merged with a similar program into the joint Banneker & Aztlán Institute, which also targets Latin and Native American students.

In addition to research, the institute emphasizes discussions on social justice issues and their relevance in the field of astronomy.