McNamara's career in law enforcement spans a thirty-five-year period. He began in Harlem as a beat patrolman for the New York City Police Department. He rose through the ranks and in midcareer was appointed a criminal justice fellow at Harvard Law School, focusing on criminal justice research methodology. Following this appointment he took a leave from police work and obtained a doctorate in public administration at Harvard. Returning to duty with the NYPD, he was appointed deputy inspector in charge of crime analysis for New York City.

In 1973 McNamara became police chief of Kansas City, Missouri, leading that department into groundbreaking research and innovative programs until 1976. In 1976 McNamara was appointed police chief for the city of San Jose, where he remained until his retirement in 1991. During his tenure, San Jose (the third-largest city in California and the eleventh largest in the United States) became the safest city in the country, despite having the fewest police per capita. The San Jose police became a model for innovation, community relations, utilization of technology, and productivity. The department's advanced training and computerization programs have been duplicated throughout the world.

McNamara has served as lecturer and adjunct professor at five different colleges and has lectured at many of the nation's top universities, including Harvard, Stanford, and the University of California at Berkeley. In 1980, he was appointed by the U. S. attorney general to the advisory board of the Bureau of Justice Statistics.

He has been a commentator for National Public Broadcasting radio and has appeared on Meet the Press, Good Morning America, the Today Show, CBS Morning News, the NewsHour with Jim Lehrer, Crossfire, Nightline, Oprah, Donahue, Larry King Live, Sixty Minutes, and other programs.

He has been a consultant for the United States Department of Justice, State Department, the Federal Bureau of Investigation, and some of the nation's largest corporations. Over the past decade, McNamara has organized four conferences at the Hoover Institution, attended by police chiefs and command officers, focusing on U. S. drug control policies.

McNamara has written five books, including three national best-selling detective novels and a respected crime prevention text. He has published articles in the New York Times, the Los Angeles Times, the Wall Street Journal, the Washington Post, the Boston Globe, the Kansas City Star, Newsday, Harper's, Cosmopolitan, National Review, USA Today, the San Francisco Chronicle, the San Jose Mercury News, and other publications. In addition, McNamara is sought as a lecturer throughout the country.