Under the leadership of District Attorney
Frank Hogan, Tanenbaum learned about trial preparedness and presenting evidence to a jury as an Assistant
New York County District Attorney in Manhattan. Later, Tanenbaum became head of the Homicide Bureau, served as Chief of the Criminal Courts, and was in charge of the D.A.'s legal staff training program. During his time in the D.A.'s office, Tanenbaum never lost a felony case. After his tenure in Manhattan's D.A.'s office, Tanenbaum served as Deputy Chief Counsel for the
House Select Committee on Assassinations to investigate the
John F. Kennedy assassination and the
Martin Luther King Jr. assassination. He later resigned from the post shortly after being named. In 1988 he appeared in the documentary
The Men Who Killed Kennedy and on 17 September 1996 he testified at a public hearing of the
Assassination Records Review Board in Los Angeles. He wrote the introduction to
Mark Lane's 2011 book
Last Word: My Indictment of the CIA in the Murder of JFK. He was interviewed for the 2023 documentary
JFK: What the Doctors Saw. In 2025 he published the book
That Day in Dallas, featuring an introduction by
Robert J. Groden. In private practice, he was a special prosecution consultant on the
Hillside Strangler case in Los Angeles; defended
Amy Grossberg in her sensationalized baby death case; and represented eight black plaintiffs in a significant racial profiling case against the
Beverly Hills Police Department. He taught Advanced Criminal Procedure for four years at Boalt Hall School of Law at the University of California, Berkeley. He conducts continuing legal education seminars for practicing lawyers in
California,
New York and
Pennsylvania. He was a member of the State Bars of New York, Pennsylvania and California. ==Political career==