My grandpa worked on the Manhattan Project, and while he died decades ago, he always maintained that his role on the project amounted to pencil pushing. But after the war, he immediately transitioned to a civilian career where he was highly sought after as an airline weights and balances engineer and was something of an expert in how to cram dangerous shit into a plane without things going awry. Supposedly, according to family lore, Grandpa was one of the masterminds who figured out how to get the bomb off the ground, into the air, and back onto the ground. But he was a notoriously tight lipped man with a litter of children with overactive imaginations, and I think he was probably telling the truth when he said he wasn't much more than a pencil pusher. I can't imagine, just based on the fact that he was in his late twenties when the bomb dropped, that he had any real responsibilities. But he was also highly educated before being recruited for the project and immediately found employment that a pencil pusher would seem woefully unqualified for. Are there unclassified employment records, or something else that would help me figure out what Gramps was up to?
Some 500,000 Americans worked on the Manhattan Project in some capacity, often without knowing what their ultimate "product" was meant to be. So while there are copious records of what people did, tracking down any individual's records can be very difficult. The first step is to figure out what site(s) they worked at. That can tell you where the relevant records might have been — e.g., the records for Los Alamos employees are not in the same place as those of Oak Ridge, or Hanford, or Dayton Project, or what have you.
If you PM me his name I can run his name through my various sources. Occasionally when people do this I actually can find something, if the person worked in a sufficiently high rank (those are the names that are easiest to find). For people lower on the totem pole, though, I don't usually have much. But I'd be happy to take a quick look.