If they were an adult male between 1917 and 1945, then their occupation and employer will be listed on WWI and WWII registration cards. All adult males had to fill one out, even if they were 70.
Check the following too:
* Death certificates.
* Land documents (not as common, but will specify farmers)
* Tax records
* Immigration records (even manifests)
* Civil war pensions (sometimes)
* Obituaries
Honestly, there are very few records that do not record occupations, at least back in the day anyway.
As for finding out who they were working for, or where, that's much more difficult. Especially for older research. As mentioned WWI and WWII registrations will tell you who they worked for, but immigration won't since they're just moving here. Civil War pensions won't, or if they do it would be rare. Obituaries might if they worked for the same employer for a long time. Tax records probably will not, unless they're post 1925, give or take a few years. It's doubtful that land records would have place of employment, but if they had to check on something they may.
There are some long-standing businesses that still have old employment records, but those are very few and far between. Many times there were no records at all, some were just day wages. Even today employers are only required to keep seven years worth of records at one time, and they can (and do) destroy records that aren't in that 7-year range.
Good luck!