John Cannon is an agent with American Military Intelligence.
There is no more detailed information about whom he answers to except that the orders for his activities comes from the Pentagon and his name and reputation are not only well known there, that knowledge extends to the White House and the Commander-in-Chief.
The time period for the actions of Cannon (in standard military style, his last name alone is usually when discussing him) is the late 60s. The Cold War is still very much a major factor as well as the expansionistic goals of both the Soviet Union and Red China.
While much of the history of Cannon before the first recorded adventure would go unspoken in those pages, we are given a brief but important glimpse into his past not too deep inside the first adventure, recapped well in a prologue to a collection of those stories, namely:
"This is Cannon, secret agent and expert in all forms of combat. He began his career as a U-2 pilot and was shot down over Red China. Captured by the Reds, he was sent to a brain-washing camp and turned over to 'Madame Toy" who made him the subject of a fantastic experiment, the most complete brainwashing ever attempted. Finally, after long months of intensive conditioning, they succeeded! John Cannon had been turned into the perfect assassin. A creature with no identity of his own. He was returned to the U.S. to kill the top nuclear scientists. But, fortunately, he was recognized and apprehended before he could accomplish his mission. All attempts to undo the brainwashing were futile - it was too complete - and so they followed the only course left to them, total reconditioning, and succeeded!"
What exactly that reconditioning involved is not known but it looked, in the image available to us, quite intensive and the man administering it was noted saying, "Forgive us, Cannon!" Also, "The result was Cannon as he is now ... something more - and less - than human",
In the first adventure we get an interesting glimpse into the attitude towards this 'new' Cannon in a comment from someone in the Pentagon who remarks that though the mission Cannon is on looks "suicidal", it is "for a man, yes, but Cannon is not just a man ... he's more like a machine" to which another person adds, "He's too [expletive] efficient! ... Gives me the creeps!"
The Cannon that the rest of the adventures depict is not really that creepy but he definitely is very, very, efficient, as Madame Toy will be forced to admit.