Barbara Gainer Johnson was admitted to practice law in the state of Illinois, the U.S. District Court, Northern District of Illinois and the U.S. Court of Appeals, Seventh Circuit in 1980. Her practice is concentrated in the area of state and local taxation. She was formerly a partner in Hynes & Johnson. She has served as a lecturer on real property taxation matters on behalf of the American Bar Association, and before business and civic groups.

She has an extensive practice in real property taxation in numerous jurisdictions throughout the state of Illinois. She is a member of the Chicago Bar Association (Real Estate Taxation Committee, Assessment and Appeals Subcommittee), the Illinois State Bar Association, the International Association of Assessing Officials and the Institute of Professionals in Taxation.

She served for ten years as a member of the Zoning Board of Appeals of Western Springs, Illinois.