Colonel Lawrence Marley is an agent with American Intelligence.
The two recorded adventures that exist of this man's activities have his employer written as being the Secret Service but this is, I believe, more a reference to the overall intelligence community than the actual then-Treasury Department organization.
Both missions we have of him take place in the "near future", each three years ahead of when they were penned. As the blurb on the first of the two puts it, "A novel of diplomacy and atomic warfare and anti-Communism, published in 1947, but speculating on possible world events in 1950 and the problems of atomic warfare". The second follows the same concept but two years later.
When we first meet Marley he is having difficulties with Jocelyn, a lovely lady he has been seeing for quite some time but who has tired of his constantly disappearing for weeks on yet another mission for the government. She worries as to how she is about to turn 27 and will, "in a few years" be old. She wants him to give up his "secret service work" of "playing the game of a Dr. Jekyll and a Mr. Hyde" and being a Sherlock Holmes. He laments that he cannot because he has just received orders from the President himself, brought about because of intel learned during his last mission.
Marley is likely retired from his Army service, or at least seconded to his intelligence work. He is 42 years old and said to be a tall man with a "bony, wind-burned face" having "broad shoulders and [an] athletic leanness". He sports scant brown hair, a trifle gray at the temples with a firm mouth, keen gray eyes, aquiline nose and a prominent chin "which gave a stamp of distinction to his clear cut face". He is said to have "the face of a man who could be a loyal friend or a ruthless enemy - a climber of the heights and an explorer of the depths".