I've always thought it should be "he is the President of the United States."
But I have seen several places where it is written "Donald Trump is P(p)resident of the United States." E.g. here and here. I thought proper noun or not, the definite article should precede the title, since there is only one office in the context. Why can the definite article be dropped?
Similarly, I have also seen sentences like "She was treasurer of the club." For instance Merriam Webster has an example "She is treasurer of the college." But examples with the definite article also abound. Like "The EVPT is the treasurer of the MIT Corporation" and "Swan is the Treasurer of a resource-rich country." Why can the definite article be dropped? If the reason is a plurality of treasures, shouldn't it be "a treasurer"? Which is the common/technically correct way?
Best Answer
I strongly dislike dropping the definite article in either of your examples. Unfortunately, Americans (I don't know about the Brits) really love to drop indefinite and definite articles whenever they can get away with such laziness. And there's really nothing an old curmudgeon like me can do about it.
What's the technically correct way? As an English language learner, err on the side of using the definite article. It's never wrong in examples similar to yours, so no one will look at you funny. Whereas, if you leave it out in places where a native speaker wouldn't, that might not be true.
What's the most common way? I don't know. 50/50?