Johnny Walker is an agent with Interpol.
Well, he is when we first meet him. Before that he was with the CIA. And then he maybe wasn't or he could have still been. It gets a little tricky. Mostly it seems Johnny Walker is an adventurer and who he works for at the time just helps out.
In that first adventure, well, when we first meet him, Walker is working for Interpol in Africa. He is just shy of 50 years old and maintains a grizzled appearance; for cover he would tell you but also because in many ways he, as the expression goes, was "rode hard and put away wet". But he does work for Interpol and he does get the job done.
The next adventure we have of him is quite a few years beforehand. It is really a two-parter with the first just a few years after the CIA was founded and Walker is one of their agents and a man who has been trained to kill and is good at it. He does not particularly like it and he talks later about sometimes he actually cried afterwards but he still did it if ordered to. That is what we learn in the first half and in the second part we find that after he quit the Agency (or got canned) and spent a half decade drunk to forget, he sobers up and starts a new life.
That is when we meet him as Frank Thomas, a new name for a new life. Married and happy but now having bad dreams as the past will not let go. But even that new life will not stay long and soon we have Walker back with the Agency and then, eventually, with Interpol. And the places he goes to will vary as much as his life - Northern Africa, Southern Africa, the Middle East, South America. Walker will get a lot of stamps in his passport.
And a lot of adventures under his belt.