Hi,
John went into his office, sat down, and did his work.
Does did his work sound good? If not, what about did his business/chore/job? Are there more alternatives?
I think it would be more natural to say John went into his office, sat down and worked.
'Did his work' subtly suggests 'all his work', but when you have a job your work is never finished. There is always more to do tomorrow.
( I'd omit the comma after 'down'. )
Best wishes, Clive