Rex E. Johnson was born and raised on a ranch north of Wheatland, Wyoming. He attended the University of Wyoming, and received his undergraduate degree in Economics in 1977. Rex graduated from UW Law School in 1980 and was admitted to the practice of law in Wyoming that same year. He is also admitted to practice in the U.S. District Court for the District of Wyoming and the U.S. Supreme Court.

Rex is a member of the Platte County Bar Association (acting as President from 1990-1996) and the Wyoming State Bar. He served on the Character and Fitness Committee of the Wyoming Board of Law Examiners for fifteen years beginning in 1998; serving as chairman for ten years; his term ended on December 31, 2012. 

Rex's practice focuses on industrial siting and condemnation, water law, corporate law, and general litigation. His primary clients are involved in electricity generation, coal transportation and coal mining.

In addition to his law practice, Rex is involved in his family’s farm/ranch operation. He married his wife, Peggy, in 1980 and has two sons.