Based on
Deathlock,
Countdown and later
The Human Factor, I assumed that Pete was in active military service, probably in military intelligence, where he reached the rank of Colonel. He would have been 21 in late 1951, so he could have seen service in Korea prior to Viet Nam; or he might just have done Cold War-related intelligence work. Based on
The Human Factor, I’d surmise that he ended up thoroughly disillusioned with the military (since he was intelligent, competent, and gave a damn about wasting lives, it would have been inevitable), and left the military to work in government intelligence after the US was out of 'Nam (1975).
I can easily imagine him having been recruited by the DXS (which is based on the CIA, which is part of the US government), and it would have been a reasonable career move at the time, since the Cold War hadn’t let up. In the
Partners flashbacks (1980), he was working as a field operative, so he’s been consistent in preferring to remain close to the action and probably resisted advancement that would have banished him to desk work.
From the DXS, the Phoenix Foundation could have actively recruited him in turn, and Mac followed him to his new employer. Phoenix may have offered him the chance to get farther away from the military and from government bureaucracy, to work more on social issues and scientific research instead of the amoral world of government intelligence and political intrigue, and possibly to remain closer to field work in spite of being in what should have been a command role.
As for Murdoc: if Pete left the military for the DXS in, say, 1975, he might have run into Murdoc starting at the beginning of that phase of his career – which would make their history particularly telling.
As for Pete’s personal background, I like to imagine that he’s third or fourth generation Irish-American, hails from the South Side of Chicago, and that his father was a cop. (Rocky, you already know I have an overactive imagination.)

With his lively career path, the overseas work, and his character having been established as a workaholic, it’s no wonder Pete didn’t have good family ties; what surprises me is that he got married at all.