Dave embarked on his professional journey as a prosecutor in the Denver District Attorney's office. Subsequently, he transitioned to the US Attorney's Office in Denver, where he assumed the role of Coordinator for the Mountain States Drug Task Force. This position was part of the newly formed Organized Crime Drug Enforcement Task Force in 1982.

In 1985, then Governor Lamm appointed Dave as the District Attorney for the First Judicial District of Colorado, encompassing Jefferson and Gilpin Counties. After being elected in 1993, he continued to serve as the District Attorney until 2005, when term limits concluded his tenure.

Governor Romer appointed Dave as the Executive Director of the Colorado Department of Public Safety in 1987, a position he held until his election as District Attorney in 1993. During his time in this role, he participated in the Harvard University Executive Course for Senior Government Executives.

For a three-year period starting in 2005, Dave assumed the position of Executive Director of the District Attorney's Council. Within that time frame, he was also designated as the Acting District Attorney in the Fifth Judicial District, which encompasses Huerfano and Las Animas Counties. This appointment was made by the Chief Judge for the Fifth Judicial District.

In 2008, Dave joined Defense Attorneys Maureen O'Brien and Fred Bibik, practicing criminal defense until 2022. In October 2022, he formed a professional partnership with his daughter, Attorney Carrie Kollar. Throughout his career, Dave has gained extensive experience in litigating various criminal cases, including sex crimes, DUIs, drug offenses, murder cases, crimes against children, felonies, traffic violations, and financial crimes.