Mark Strong is an agent with British Intelligence.
More specifically he is an operative with a division of that organization known as 'J' Branch. It operates out of an office "on the eighth floor of a recently completed office block in the heart of London. The brass plate on the door said "British Travel Agents, Ltd." Interestingly, it is run by a man who was known only as 'J'.
The department has some very relaxed attitudes towards legalities. A case in point is in the first recorded adventure when a man in Paris with needed intel but a reluctance to divulge it is drugged to sleep and transported secretly back to London for an interesting session of threatened torture.
Strong is mentioned several times as being the ace operative for this group and shows no evidence of changing his winning ways. He is not afraid to leap into enemy territory or walk boldly into a hostile embassy, confident he will live long enough to survive getting back out again.
He is a tall, ruggedly good-looking man with somewhat wavy blond hair brushed straight back.
He has a nice set of devices in his arsenal, curtesy of 'J' Branch's inventiveness. An example is a radio transmitter and receiver small enough to fit in his suit pocket but said to be the powerful one in the world. Another is a powerful fast-acting knock-out gas he carries in a vial in the heel of a shoe.
In the first mission we are able to follow him on, he meets a foreign spy chief named Golen who will become a personal nemesis. That man is from the European country of Krania, an obviously Eastern European nation with a major dislike of the U.K.