Joe Stanford is an agent with the CIA.
This fact is certain but from that point on, the history of Stanford becomes a bit confusing and possibly contradictory. The first adventure takes place in 1968, nearly two and a half decades after the end of WWII. At the beginning of that tale, Stanford muses somewhat about his age, which is 46, and his life, sketchy though that musing is. He mentions having served in the OSS which, since it dissolved in 1945, meant he was in the Second World War. Considering his age, he would have been in his early 20's then.
He also recalls how he had served with some distinction with the U.S. Army Special Forces. This band of highly skilled warriors was founded in 1952 which would have made him in his late 20's. Though it is not stated, it is logical to assume that he was one of the first to wear the Green Beret since had he waited much longer, he would have been in his 30's and hardly a desired candidate for such a rigorous detachment. At any rate, he had been so serving for many years and had reached the rank of major. Much of that time had been seconded in one capacity or another to the CIA, again a logical connection considering his OSS origins. It is interesting, though, that Stanford, after over two decades of service, had only reached the O-4 level.
That mystery aside, it is recorded that during the years between the end of WWII and the thickening of action in Vietnam, Stanford had become a cold-blooded killer who was extremely gifted in his ability to eliminate without hesitation. There is no stated tally as to his targets but comments made by a superior indicate it was not small. At one point, Stanford had started to wonder if he was still a member of the human race, so callously was he able to take another's life.
It was in Vietnam, though, that humanity came back to him when saw so much killing and destruction that he started to hate it and to seriously distrust all the people who gave the orders to have others killed for no obvious reason. He did not quit his work but he did begin to question it and it is possibly that which explains why he never made colonel.
Once the two-book series begins, Stanford's life takes a dramatic and permanent change. In the first book he is asked to have his face altered to look closer to another agent reportedly dead in action. He assumes that identity so completely that for most of the adventure, he is referred to, and often comes close to thinking of himself as, that replaced agent. As the book nears the end, his life is in great danger both as the assumed identity and, he learns to his disgust, as his original one. Someone wants that other agent dead and wants the man replacing him to be dead as well.
The second adventure comes shortly thereafter when he has started hoping his previous life, or lives, are behind him and he is starting again, now in his early 50's and tired of all the killing. Defending himself, though, is still an instinctual action and he shows both himself and those coming after him that just because one is older does not mean one is over.