In 1934, Banister joined the Federal Bureau of Investigation (FBI). Originally based in Indianapolis, he later moved to New York City where he was involved in the investigation of the American Communist Party. J. Edgar Hoover was impressed by Banister's work and in 1938 he was promoted to run the FBI unit in Butte, Montana. He also served in Oklahoma City, Minneapolis and Chicago before he retired from the FBI in 1954.
Banister moved back to Louisiana and in January 1955 became Assistant Superintendent of the New Orleans Police Department where he was given the task of investigating organized crime and corruption within the police force. It later emerged that he was also involved in looking at the role that left-wing political activists were playing in the struggle for black civil rights in New Orleans.
Banister developed extreme right-wing views and worked as an investigator for the Louisiana Un-American Activities Committee. He also published the racist Louisiana Intelligence Digest. Banister had a deep hatred of the civil rights movement and believed that the policy of racial integration was part of a a plan formulated by Joseph Stalin to create racial conflict in America.
Bannister claimed that members of the American Communist Party were involved in a plot to contaminate crops in the United States. He also told the Special Committee of the Arkansas State Legislature that left-wing activists were behind the race riots in Little Rock.
Banister was suspended by the New Orleans Police Department (NOPD) for an incident with a gun in a bar. His suspension ended in June 1954, but when he refused to be transferred to the NOPD's Planning Department, he was dismissed from the force. After leaving the police he established his own private detective agency, Guy Banister Associates.
Later Banister was linked to the plot to assassinate John F. Kennedy. On 9th August, 1963, Lee Harvey Oswald distributed leaflets that supported Fidel Castro and his government in Cuba. On these leaflets was the address 544 Camp Street, New Orleans. This was also the office of Carlos Bringuier, an anti-Castro exile. Around the corner from 544 Camp Street, located in the same building, was 531 Lafayette Street, which housed the detective agency run by Banister. This raised suspicions that Oswald had been involved in a right-wing conspiracy to kill Kennedy.
On the afternoon of 22nd November, 1963, Banister and Jack Martin went drinking together. On their return to Banister's office the two men got involved in a dispute about a missing file. Banister became so angry that he he drew his Magnum revolver and hit Martin with it several times. Martin was so badly injured that he had to be detained in the local Charity Hospital.
Over the next few days Martin told friends that Banister and David Ferrie had been involved in the assassination of John F. Kennedy. According to Martin, Ferrie was the getaway man whose job it was to fly the assassin out of Texas. He also claimed that Ferrie knew Lee Harvey Oswald from their days in the New Orleans Civil Air Patrol and had given him lessons on how to use a rifle with a telescopic sight.
On 25th November, Martin was contacted by the Federal Bureau of Investigation. He told them that he thought Ferrie had hypnotized Oswald into assassinating Kennedy. The FBI considered Martin's evidence unreliable and decided not to investigate Banister and Ferrie.
This information eventually reached Jim Garrison, the district attorney of New Orleans. He interviewed Martin about these accusations. Martin claimed that during the summer of 1963 Banister and David Ferrie were involved in something very sinister with a group of Cuban exiles.
Jim Garrison now became convinced that a group of right-wing activists, including Banister, David Ferrie, Carlos Bringuier and Clay Shaw, were involved in a conspiracy with the CIA to kill John F. Kennedy. Garrison claimed this was in retaliation for his attempts to obtain a peace settlement in both Cuba and Vietnam.
Delphine Roberts worked for Banister and later became his mistress. Roberts told Anthony Summers that during the summer of 1963 Lee Harvey Oswald worked for Banister. She said she was in the office when Banister suggested that Oswald should establish a local Fair Play for Cuba Committee. This story was supported by her daughter who met Oswald during this period.
Guy Banister died of coronary thrombosis on June 6, 1964.