Bart Gould is an agent working for the President.
Gould is a rich playboy who has done, mastered, and then grown bored with many of the excitements that interest people of daring. He has raced at LeMans, climbed the tallest mountains, hunted big game, and so on, and has the scars to prove it. His action reveal a recklessness to match his talents making him open to just about any challenge.
He had not always been so. Born into a rich and powerful family of businessmen and politicians, Gould had proven himself to be both intelligent and brave as he followed in his heritage of service to the country by becoming an officer in the U.S. Marines. It was in this job that he earned numerous medals, culminating in the Congressional Medal of Honor.
When the woman he was to marry died in a plane crash when Gould was 26, his life and his attitude towards it changed. Far more abandon came into his actions as he drove himself harder and harder to find a purpose to fill the void her passing left. In doing so, he soon learned that once you've done just about everything most anything seems a bit boring. Which is why he responds when he gets a summons to the White House.
Through a very close friend to the President he learns that the Chief Executive, who has known Gould since he was young, needs a highly trained operative to go places no agent of the government could openly go to do things that cannot be officially done. Considering Gould's experience and talents, plus the jet set reputation he also enjoys, he is the first and only choice.
Now, at age 36, he is the President's Agent, a man with total deniability working with highly secret backing to do the impossible.
Note that the first book in the series was published as being by Joseph Hilton, not Joseph Milton as the remainder were. According to Hubin's Crime Fiction IV, Hilton was a pseudonym of Joseph Hilton Smyth while Milton was a house pseudonym for several authors, only two of which have been identified.