Captain Maddox is a member of the British S.O.E.
That acronym stands for the Special Operations Executive, whose "purpose was to conduct espionage, sabotage and reconnaissance in occupied Europe (and later, also in occupied Southeast Asia) against the Axis powers, and to aid local resistance movements." The exploits of the members of this organization, where not kept classified, have been extremely impressive and that holds true with Maddox.
Maddox, when we first meet him is an experienced agent. Since the S.O.E. was created in 1940 and the recorded adventures we have of Maddox start in 1944, he has obviously been around for several undoubtedly active years.