Punctuation Q

[vcolts]

Ex:

Paul was hired as a general manager last fall, and still works as a general manager.


Question:

Should I be putting a comma before the conjuction in sentences like the above? (Academic/formal writing wise)
I learned and still see that the grammar books indicate that you cannot put a comma there if the following is a phrase/does not have a subject.

Is it because we treat the omitted subject "he" as existing grammatically and treat the second part as a full sentence?

Thanks in advance.

[JohnParis]

To avoid any confusion, make two sentences.

Last fall, Paul was hired as a general manager. He is still a general manager.