What do you call a person who started something that is later followed by everyone?

A trend-setter or trendsetter:

someone who starts a trend, or makes one more popular

The other answers on this page have a couple other fine suggestions — and a thesaurus lookup turns up more suggestions still —, but be aware that some of them require additional quali­fication. For example, it's typically "a harbinger of something", or "a precursor to something". So you won't be able to just say "he is a harbinger" and leave it at that; likewise, "he is a pre­cursor" will only raise questions if no further context is supplied. "He is a trendsetter", on the other hand, is perfectly self-explanatory and self-sufficient.

Pioneer is the word for that, but in this particular example, it may not be a good fit. Pioneer carries a positive emotion, and I would not use it for something perceived as negative (like staying late at work).

I also rather like forerunner for someone who starts a trend — sometimes even harbinger or herald, depending.

The thing about harbinger and herald is that those portend (or augur) something upcoming if not imminent, while a forerunner is just ahead of his time.

Maybe also initiator could fit?

one who initiates