I believe you may be remembering some aspects of 'Through the Gates of the Silver Key'; you've somewhat described the ending of it.
It's the final part of the main Randolph Carter sequence; The Dream-Quest of Unknown Kadath, The Silver Key and the aforementioned Through the Gates of the Silver Key.
If you would like to re-read it to confirm, it's passed it's copyright age, and can be read freely at a number of places; the links above will take you to the Wikisource copies. (One caveat; given when it was written, and HP Lovecraft's known prejudices, it's not a very politically correct work, and some of the language may be offensive to some.)
However, stealing from the Wikipedia Synopsis:
After a vast amount of time trapped on Yaddith, Carter finds a means of suppressing the alien mind with drugs, and then uses their technology, along with the Silver Key to return both to the present and to Earth, where Carter can retrieve his manuscript with the symbols he needs to work on regaining his original body. Once there, the Swami reports, Carter did find the manuscript and promptly contacted Swami Chandraputra, instructing him to go to the meeting to say he would soon be along to reclaim his estate and to continue to hold it in trust. After the Swami finishes the tale, one in the party, the lawyer Aspinwall (who is Carter's cousin), accuses Swami Chandraputra of telling a false tale in an attempt to steal the estate, claiming that he is some kind of conman in a disguise. As Aspinwall tears at the Swami's masklike face and beard, it is revealed that the Swami is not human at all, but Carter, still trapped in Zkauba's hideous body
Near the end, the unmasking you refer to occurs:
"Hey, by Heaven I've got it! This rascal is in disguise. I don't believe he's an East Indian at all. That face - it isn't a face, but a mask! I guess his story put that into my head, but it's true. It never moves, and that turban and beard hide the edges. This fellow's a common crook! He isn't even a foreigner - I've been watching his language. He's a Yankee of some sort. And look at those mittens - he knows his fingerprints could be spotted. Damn you, I'll pull that thing off -"
Does that ring a bell?