Marrs earned a Bachelor of Arts degree in journalism from the University of North Texas in 1966 and attended Graduate School at Texas Tech in Lubbock for two years more. He has worked for several Texas newspapers, including the Fort Worth Star-Telegram, where, beginning in 1968, he served as police reporter and general assignments reporter covering stories locally, in Europe, and in the Middle East. After a leave of absence to serve with a Fourth Army intelligence unit during the Vietnam War, he became military and aerospace writer for the newspaper and an investigative reporter.

Since 1980, Marrs has been a free-lance writer, author, and public relations consultant. He has also published a rural weekly newspaper along with a monthly tourism tabloid, a cable television show, and several videos.

Since 1976, Marrs has taught a course on the assassination of President John F. Kennedy at the University of Texas at Arlington.  In 1989, his book, Crossfire: The Plot That Killed Kennedy, was published and reached the New York Times Paperback Non-Fiction Best Seller list in mid-February 1992.  It became a basis for the Oliver Stone film JFK. Marrs served as a chief consultant for both the film’s screenplay and production.   

Beginning in 1992, Marrs spent three years researching and completing a non-fiction book on a top-secret government program called the Stargate Project involving the psychic phenomenon known as remote viewing, only to have it canceled as it was going to press in the summer of 1995. Within two months, the story of military-developed remote viewing broke nationally in the Washington Post after the CIA revealed the program.

In May 1997, Marrs’ investigation of UFOs, Alien Agenda, was published by HarperCollins Publishers. Publishers Weekly described Alien Agenda as "the most entertaining and complete overview of flying saucers and their crew in years". The paperback edition was released in mid-1998. It has been translated into several foreign languages and become the top-selling UFO book in the world.

In early 2000, HarperCollins published Rule by Secrecy, which claimed to trace a hidden history connecting modern secret societies to ancient and medieval times. This book also reached the New York Times Best Seller list. In 2003, his book The War on Freedom probed the alleged conspiracies of the 9/11 attacks and their aftermath. It was released in 2006 under the titleThe Terror Conspiracy.

Marrs has been a featured speaker at a number of national conferences including the annual International UFO Congress and the annual Gulf Breeze UFO Conference,  but he also speaks at local conferences, such as Conspiracy Con and The Bay Area UFO Expo.   Beginning in 2000, he began teaching a course on UFOs at the University of Texas at Arlington.

Marrs has appeared on ABC, NBC, CBS, CNN, C-SPAN, The Discovery Channel, TLC, The History Channel, This Morning America, Geraldo, The Montel Williams Show,Today, TechTV, Larry King (with George Noory), and Art Bell radio programs, as well as numerous national and regional radio and TV shows.