Tiger Mann is an agent for a secret organization.
As simply put as I can, take the private investigator Mike Hammer and give him a job working for a multimillionaire right winger and you'd have Tiger Mann. The epitome of hard drinking, hard fighting, hard loving action hero writers took a break from his detailing the adventures of the aforementioned Hammer to quickly pen four missions of Tiger Mann in two years.
In 1964, the Western world was quickly becoming spy-happy. Two Bond movies had been released and the third, Goldfinger, was coming out that year. On television, The Man From U.N.C.L.E. was starting its first season. And in the publishing world, the golden area of spy-fi was just starting (more new series were started in 1964-1966 and any other 3-year period). Apparently answering his agent's suggestion, Mr. Spillane joined the throng.
Mann was a survivor of WWII, having worked in the OSS during the final years. After the war, he was recruited by his boss to help in a private organization to keep liberty alive in the wake of newly perceived danger from Communism. While that group was not given a name, its presence was felt by many nations as it stepped in frequently when it seemed governments were reluctant to act decisively enough.
According to comments made by Mann throughout the chronicles, he and his small band of associates had helped several small governments remain in power while assisting several others in leaving the scene, all for the furtherance of the American way. As he goes about the tasks assigned him, he often runs up against legitimate intelligence agencies but the influence that his boss has with the powers-that-be usually clears the way.
FYI, the first name of Tiger was not a nickname. By his own words, it was his real name given him by his father.