In 1961, Kornblum joined the New York City Police Department before being drafted by the U.S. Army. He served in Korea and upstate New York before separating and joining the Federal Bureau of Investigation. During his time in the FBI, Kornblum worked on Civil Rights cases in Mississippi and bank robbery cases in New York City. From there Kornblum moved to Princeton University to become the Director of Security while simultaneously earning a PhD, writing a dissertation on corruption in the New York City Police Department.
Kornblum was recruited by the Department of Justice to help reign in U.S. Government surveillance operations in the mid-1970s. He wrote key parts of the Foreign Intelligence Surveillance Act (FISA) and oversaw the law's implementation for the next two decades. In particular he developed the policies for minimization; that the Government should only retain valuable intelligence information and not collect other private information. During this time Kornblum was involved in the investigations of such notable spies as John Anthony Walker and Aldrich Ames.[2]
In 2003, Kornblum testified at a federal trial based on his experience with the FBI in Mississippi. Ernest Avants, a former Ku Klux Klan member, was accused of killing Ben Chester White in 1967.[4] Kornblum testified Avants confessed to the crime, "'Yeah, I shot that nigger,'", but Avants added that when he shot White, another man had already shot him several times. Kornblum noted Avants' attorney advised him he would be acquitted as "you can't be convicted for killing a dead man." Avants was convicted and later died in prison.[5]
Later that same year, Kornblum was appointed as a United States magistrate judge for the Northern District of Florida. He assumed his office on October 17, 2003, and served until his death seven years later.