Every public employee is listed in the Official Register, which was published every two years.
Here's a link to on-line copies of these:
https://catalog.hathitrust.org/Record/009557655If you go a few pages into each one, you'll find the index. Under "Post Office," there is an entry for Postmaster General. On that page, you can find the names of the Postmaster, 2nd Asst. Postmaster, 3rd Asst. Postmaster.
For example, here's 1867:
https://babel.hathitrust.org/cgi/pt...=1up;seq=356Every local postmaster is also listed for every office in the country.