Mark Pierce is an agent with the CIA.
The nickname for Pierce, and for this series of thrillers as well, is The Trigger Man". He has earned this sobriquet while a paramilitary officer with the CIA Special Activities Center's Special Operations Group, (SAC/SOG), but not for the reason that many apparently believe. As one person corrected, it was not because he was "some super-sniper who's taken out hundreds of Afghani and Iraqi insurgents" (though the speaker did not say he had not) but because "if you want to 'trigger' a volatile situation, shake things up, he was the man to do it".
On another occasion, someone opined to Pierce that "much of your past is a highly classified secret but I'm guessing something traumatic happened during that redacted period. You were hurt so bad, you trust no one." Pierce did not voice any opinion to either part of that statement.
It is very possible that 'Mark Pierce' is not even the man's real name; that part being made somewhat vague in an early exchange between he and DGSE agent using an obvious phony name.
His past is definitely shrouded in secret as we learn from his current handler, a woman named Mackenzie Summerfield as she states, "Pierce joined us already trained in covert operations and tradecraft. He must be ex-special forces. He speaks all five MASER languages, plus some others. He is particularly adept at being on his own, taking care of his own problems. This has led to some issued with higher-ups -- Summerfield is his fourth handler and the only one to last more than a month. As she puts it, "he follows unconventional routes to solutions that don't always seem obvious at first assessment". Even more telling and considerably more interesting, "he's so used to running his own missions, he no longer knows how to follow the rules".