After graduating from Washburn he became an assistant public defender. Rosen later served as an assistant district attorney in Shawnee County, Kansas, and eventually took the office of associate general counsel to the Kansas Securities Commission before entering the private practice of law in 1990. In 1993 Rosen took his first judgeship on the District Court for Shawnee County. He was appointed Chief Justice to the Kansas Sentencing Commission in July 2002. During this time Rosen was a member of Koch Crime Commission, lectured at the Menninger School of Law and Psychiatry, 2004-2005 President of the Sam A. Crow Inns of Court and was appointed to the presidential commission charged with commemorating the 50th anniversary of the Brown v. Board of Education decision.