Dr. Mahr was born and raised on a dairy farm in south central Iowa. He received his veterinary medical degree from Iowa State University in 1971. After working as an associate in suburban Chicago for three years, he established, owned, and directed the Meadow View Veterinary Clinic in Geneva, Illinois, an accredited hospital member of the American Animal Hospital Association from 1974 to 2005.

Dr. Mahr has been active at all levels of organized veterinary medicine his entire career. He served as a member of the Board of Directors and as President of the Chicago Veterinary Medical Association. A member of the Illinois State Veterinary Medical Association (ISVMA) since 1971, he served as chairman of their Membership and Annual Convention Scientific Program Committees, on the Board of Directors, and served as ISVMA President in 1999. Dr. Mahr also served as President of the Humane Society of North Central Illinois and as a member and chair of the Illinois Veterinary Licensing and Disciplinary Board.

On the national level, Dr. Mahr served as a member and chair of the American Veterinary Medical Association (AVMA) Executive Board representing District VI (Illinois, Indiana and Wisconsin). He served on the AVMA's Long Range Planning Committee and Committee on Environmental Affairs. He chaired the Model Veterinary Practice Act Task Force and served a three-year term on the Board of Directors of the American Veterinary Medical Foundation. He is also a member of the American Association of Bovine Practitioners, the American Association of Food Hygiene Veterinarians, and the AVMA PAC Presidential Club.

Dr. Mahr was elected unanimously to the office of President-elect of the AVMA, and pledged to focus on improving animal and human health and increasing the number of veterinarians entering the areas of food safety, biomedical research, academia, regulatory medicine and emergency health preparedness. As AVMA President he revealed a vision for a one health initiative whereby the veterinary profession takes the lead to establish a coordinated mechanism to facilitate collaboration and cooperation among colleagues in veterinary medicine, human medicine, public health and environmental sciences with a focus on "One World, One Health, One Medicine".

Dr. Mahr is active in community and civic affairs. He is a member of the Delnor Community Hospital Foundation, and is a member and former president of the Geneva Lions Club. He served as a member of the Strategic Planning and Facilities Advisory Committees of the St. Charles Community School District, and as a member, vice president and president of the Board of Education. He is a member of the United Methodist Church of Geneva and served on the church's Administrative Board.