Ken Croydon, aka Secret Agent M-11, is an agent with U.S. Military Intelligence.
It is never expressly said that is who he works for but the inference is strong. He can summon a plane or a ship as needed and he takes his orders from a man in uniform so it is most likely that he works for the military in the Intelligence field though he never is seen wearing a uniform himself nor is he ever addressed by any rank. In one adventure a man catching a foreign spy shouts that "this is a case for the F.B.I." but they call the "local police" who then notice when Croydon arrives and calls him by name so the possibility exists that he is really a G-Man.
He is apparently experienced and known on both sides under his codename of M-11. In one of the adventures when he shows up unannounced, the leader of the bad guys is surprised and stutters out his codename when he comes in pistol ready. Add to that the fact that no one challenges his orders and jumps when he directs and it is reasonable to assume he is not terribly covert in his activities despite having "secret agent" in his moniker.
Very little is known about M-11. We see from his depiction that he is likely in his late 30s, is in excellent shape with a sloping forehead with short black hair, possibly a bit wavy.
M-11 is quite fond of fisticuffs and uses a quick punch to the jaw to announce his arrival. He is good with a handgun and might be ambidextrous as sometimes he holds it in his right and other times his left. He must have a particularly hard head able to bounce back from all sorts of attacks because he is frequently smacked on the back of his noggin and responds quickly and decisively when it happens.
Though the United States is not actively at war when the four adventures of M-11 take place, it is most definitely under attack behind the scenes with enemy agents from one or more European nations doing what they can to either disrupt supplies to the Allies or to weaken the country so their bosses can march in and take over. Luckily for the security of the nation, M-11 is ready to take them on.