Johnston earned his Bachelor of Science, cum laude, from Rockford University in 1987 and his Juris Doctor, cum laude, from UIC John Marshall Law School in 1990.

He was a unit supervisor for the Office of the Illinois Attorney General and in private practice at Altheimer & Gray, Holland & Knight, and Johnston Greene.

He also serves as an adjunct professor at UIC John Marshall Law School.

[5] President Trump nominated Johnston to the seat vacated by Judge Frederick J. Kapala, who assumed senior status on May 10, 2019.

[7] On September 16, 2020, the United States Senate invoked cloture on his nomination by an 81–15 vote.