Karrer joined the attorney general's office in November 2008. Previously, he served as an assistant district attorney in Shawnee County and worked in private practice. He has extensive experience in prosecuting financial crimes including elder and fiduciary abuse. He is cross-designated as a Special Assistant United States Attorney, which in coordination with the United States Attorney of Kansas, allows him to prosecute in federal court in addition to state court. Karrer received his bachelor's degree from Nebraska Wesleyan University and his law degree from Washburn University School of Law.