Jed Walker is an agent with the CIA.
Is when we first meet him. Was soon after that, the result of a little disagreement mostly because in the very early pages of the first recorded adventure (so not much of a spoiler alert needed), a higher-up in the Agency decides not only is Walker's service no longer needed, he is not either. It is not surprising that almost being shot by a coworker and then almost blown up by a friendly fire missile can dampen an agent's esprit de corps.
When we encounter him, he is in his very late 30's and has been in the business of being, as it is put several times in different ways, the sharp end of the stick in America's defense of itself.
Initially there had been his graduation from the Air Force Academy and his required service to the country, which he performed admirably as part of the Air Force Special Operations Command, specifically with the highly elite 24th Special Tactics Squadron - The USAF's equivalent of Army's Delta and the Navy's SEALS. He was on active duty when 9/11 took place and decided then that when his time came up, he would transition to a more active role in the fight against terrorism.
So it was that in 2005 he joined the CIA and found numerous missions that would get him deep inside the action and give him multiple chances to eliminate enemy threats. He was good at it and, if he were honest with himself, enjoyed the excitement and the danger and probably would have stayed with them for as long as his physical body would allow.
Except for a new terrorist group called Zodiac and issues with certain people inside the CIA and some major issues with his father.