Mark Strong is an agent with M.ATT.E.L.
As we learn at the beginning in The Mark Strong Dossier, "Mark Strong is a Crimefighter (no idea why the last word was capitalized). He's the leading agent for an organization known as M.ATT.E.L., Missions Attached to Europe and London. M.ATT.E.L. is pledged to fight crime throughout the world. And in particular to smash the syndicate of criminals known as I.C.E. I.C.E. (International Crime Enterprises) is the world's leading organization for smuggling, forgery and larceny. Mark is internationally known as a racing driver, and this provides an ideal cover for his real work. He reports to H.Q., a converted toy shop somewhere in London. Behind the facade lies a complicated mechanical network of equipment. The kingpin of this is the Thinking Computer through which Mark receives his instructions, often in code."
In addition to his exceptional abilities behind the steering wheel, Strong is a karate expert, a skin diver, an athlete, and a footballer, or so we are told in that dossier, as well as a creative and logical mind which he used to come up with the Unique Coding Device, the encryption tool used to pass his messages back and forth. He has lush brown hair and hazel eyes.
Although his dossier states that he joined the bureau in 1972, in his first recorded adventure he recognizes a hoodlum driving another car and muses "the last time I saw that guy he was in prison" which indicates he likely has been around for a while. Also his familiarity with the headquarters and its procedures show experience likely needed more than just a few months.