What do you call a person who started something that is later followed by everyone?
Solution 1:
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.
Solution 2:
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).
Solution 3:
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.